Semantic change is inevitable, rarely unambiguous, and wildly unpredictable. Still, it is possible to identify specific types of word change, even if we do so after the fact. Some lexical change has to do with a shift from negative to favorable meaning (amelioration), or the reverse (pejoration). Other types of word change fall under the categories of generalization (extension) or narrowing (reduction) of a semantic field. These four processes are quite common in historical linguistics.
Two less common mechanisms of word change are semantic strengthening and semantic weakening. Strengthening and weakening have to do with the force of word meaning, with its intensity. Weakening is a much more common process than strengthening. In A Biography of the English Language (HBJ 1988, pp 181-182), C.M. Millward explains why: “In general, strengthening is rarer in language than weakening-evidence that people are more prone to exaggeration (which tends to weaken meanings) than to understatement (which tends to strengthen meanings).”
Examples of semantic strengthening include: appalled, which had an original meaning of “feeble” and “pale” until the early 16th century, when the more intense meaning of “deeply dismayed” and “shocked” appeared in the work of Sir Thomas More; jeopardy (jeu parti), which shifted from “even chance” to “risk” to “danger”; wreak, which gradually shifted from the milder meanings of “push” and “urge” to the gloom-and-doom connotations of havoc, damage, destruction, and revenge. (Notably, this stronger meaning of wreak developed as late as 1817, when it is first recorded in the writings of Shelley.)
Examples of semantic weakening are numerous: The verb quell originally meant “to kill” (Old English cwellan) and now means “to subdue” (as in quell fear, doubts, or opposition). The adverb soon (Old English sona) used to mean “straightaway”, but then the element of procrastination seeped in and weakened the immediacy of the original meaning to a milder “in a short while”. The original meaning of fret was “to devour” (cf. modern German fressen) before it weakened to mere fretting and worrying. The Old English ancestor of the verb spill (spillan) meant “to destroy, to kill”, i.e. “to shed blood”. Then, through the process of weakening, the meaning of spill shifted from spilling blood to a much less gruesome kind of spillage (as in spilling milk).
Finally, let’s consider this line, spoken by Portia, from Shakespeare’s “The Merchant of Venice”:
So shines a good deed in a naughty world.
The word naughty is used here in its original, stronger meaning of “immoral” and “evil”. In Shakespeare’s time (and, regrettably, to this day) there was evil in the world, and it is to this evil that Portia refers. The weakened meanings of naughty (“disobedient”, “improper”, “sexually suggestive”) do not apply in this context.
Merriam-Webster Online: http://www.m-w.com
A Biography of the English Language by C.M. Millward (HBJ 1988)
Shakespeare Online: http://www.shakespeare-online.com