Pejoration is the process by which the meaning of a word becomes negative or less elevated over a period of time, as silly, which formerly meant "deserving sympathy, helpless or simple," has come to mean "showing a lack of good sense, frivolous."
Other Examples:
Homely: from "characteristic of the home" to "not attractive"
Lady: from a parallel to "gentleman," it has acquired a derogatory sense
Lewd: from "belonging to the common people" to "worthless and vile"
Mistress: from a parallel to "mister," it has acquired sexual connotations
Prude: from "wise woman" to "a person who is excessively concerned with being proper"
Rape: from "seize" to "force someone to submit to sexual acts"
Senile: from "relating to old age" to "having the weaknesses of old age"
Surly: from "lordly" to "bad-tempered"
Vulgar: from "the common people" to "rude and unrefined"
