As formulated by Saussure, a linguistic sign is the combination of the sound pattern of the word (the signifier) and the mental concept which it invokes (the signified). The connection between the two is purely arbitrary, or conventional. Because there is no intrinsic connection between the sound pattern and the concept being signified, changes in either the signifier or the signified are able to occur over time. The sound pattern of a signifier can change over time through processes like assimilation, metathesis, reduction, and so on. But the concept referred to by the signifier may also change.
A current example of how the concept referred to by a particular sound pattern can change can be seen in the word "literally." For many speakers, this word means "non-figuratively," while for others it now can mean the exact opposite, that is, "figuratively" as in a sentence like “My head literally exploded.” For many people today this sentence makes no sense, since it violates the meaning they ascribe to the sound pattern "literally." Others, however, accept this new usage of "literally." simply because many people use it this way. Once you grant that the connection between the sound pattern [lɪtərəli] and the concept it signifies, whether "non-figuratively" or "figuratively," is conventional and arbitrary, then you must grant that either meaning is allowable if there is a group of speakers who have agreed upon the conventional connection between the two. In fact, this change is not even a new one: English speakers have been using the sound pattern [lɪtərəli] to signify "non-figuratively" for several centuries. We can find examples of it as far back as the 1700’s. If you are speaking from a personal point of view as a speaker of English, then you are free either to accept or not accept that usage in your own version of English. But the reason you do not accept it will not be because of any supposed violation of the word’s etymology, since we violate the etymological meanings of words in almost every sentence we utter. The same goes for other “commonly misused words” such as irregardless, disinterested, or ironic, which of course, are not “misused” at all.
Generalization and specialization (also called widening and narrowing)
It is quite common for us to “narrow” the scope of meaning of a particular word. An example for this is the word corn. In Old English it could refer to any type of grain at all, a fairly wide meaning. It was eventually narrowed to mean “wheat” specifically. Today, in the U.S., it refers only to one specific type of plant, elsewhere called “maize.” Another food term that has narrowed its meaning is the word meat, which in Old English referred to any sort of food at all (except liquids). Today, it is used to refer specifically to what was once called “flesh” (as it still is in German, Fleisch). The word "flesh" has also narrowed in some uses to refer only to the skin.
Somewhat less common is “generalization,” or “widening,” where the referents become less specific. One example is the Old English compound bere-ærn, “barley-building.” This is modern English “barn,” a word that can refer to buildings that hold much more than only barley. We also tend to take certain brand names and generalize them. “Band-Aid” can refer to any sort of adhesive bandage, and “Kleenex” can refer to any number of “tissues,” not only those made by Kleenex.
Amelioration and Pejoration
This shift is not necessarily one of a change in the denotation of the word (i.e., the literal object or concept it refers to), but in the connotation, that is, the associations or feelings the word implies: some words take on positive connotations (ameliorization), while others take on negative ones (pejoration). Take the word lewd, for example. In Old English it simply referred to someone who had not been taught to read Latin, i.e., the uneducated, and thus it referred to the majority of the population. Today, it refers to someone who is crude, often sexually. On the other hand, the word cniht in Old English meant a “boy” or even a "servant." Today, as the word knight, it has much more positive connotations of heroism and chivalry.
Intensification, Strengthening, and Weakening
The word awesome is a common example of weakening. It used to mean something that was capable of inspiring awe, and was often used to describe various deities or religious experiences. This would be true of the word today if you are describing a view of the Grand Canyon, but if you are describing the latest episode of some sit-com, the meaning has been someone weakened. Another example is the word soon, which in Old English meant “immediately,” but now could be several hours away if not longer. Many curse words have also undergone weakening. Few people today who say “Damn it” are really thinking of confining whatever object to eternal hellfire. It is much rarer to find examples of strengthening. One is the word decimate. Its root is from Latin decem, meaning "ten," and it originally meant to destroy or diminish only one-tenth of something. In fact, it often meant to tithe, which is giving one-tenth of one’s income to the church. Today, to decimate means to destroy something completely.
Transfer of Meaning
This is a large category and includes metonymy, synecdoche, metaphor, abstraction, and concretization. Synecdoche (this word has four syllables; the end rhymes with “key”) is when we refer to something by just a part of it, for example, calling a sword a “blade,” or an old person a “graybeard,” or a car “wheels.” Metonymy is similar, but instead of referring to an object by something which is physically attached to it, we refer to it by something with which it is associated. Thus, the press might report an announcement by the Pentagon or the White House, when they mean the military or the president; or someone might refer to executives as “suits.” We also use metaphor quite often in our speech; for example, we metaphorically refer to the parts of the table that hold it up as the “leg,” and we speak of the “hands” of a clock. Some words have lost their original meanings and only preserved the metaphorical one, such as "ridge," which originally referred to the back of a person but was metaphorically extended to the crest of a mountain range, since it looks like the bumps of the spine. We use "spine" to refer to the back of a book, like a person's back. Originally a spine refered to a sharp point, like the thorns of a plant, but this was metaphorically extended to refer to the sharp points of a person's backbone. Other metaphorical transfers of meaning involve abstraction, such as using the word “grasp” to mean “comprehend” a concept rather than to grab something with one’s hands. Metaphorical concretization is less common. One example would be “spectacles,” referring to glasses with which you view some type of show, rather than the show itself.
Taboo or Euphemistic Alterations
Words can often change because they are thought to refer to something that is taboo and to which therefore it is best not to refer. Common semantic fields in which this occurs are related to death, sex, and scatology. There are two common methods for dealing with taboo terms. One is taboo deformation in which the term itself is altered, such as darn instead of damn. More common is the use of a euphemism, that is, the substitution of another phrase for the taboo term. For example, bathroom, a room with a bathtub in it is a euphemism for toilet. In Old English one way to refer to a toilet was gangsetl, literally, a “seat on which one goes.” There are numerous non-literal ways to say “die” or “to have sex with.” It is in fact hard to trace the origins of a number of such words, since the monks who were the only ones to write in the Old English period apparently thought they were taboo words as well. The word “die” does not appear in Old English at all; they most commonly refer to people as “going forth” or "lying down." There are several words in Old English for sexual intercourse, but the one known euphemistically today as “the F-word” never appears even though scholars assume it must have existed then.