Semantic Shift

Semantic change (also semantic shift, semantic progression or semantic drift) is the evolution of word usage — usually to the point that the modern meaning is radically different from the original usage.

examples

Awful—Originally meant "inspiring wonder (or fear)". Used originally as a shortening for "full of awe", in contemporary usage the word usually has negative meaning

•Demagogue—Originally meant "a popular leader". It is from the Greek dēmagōgós "leader of the people", from dēmos "people" +agōgós "leading, guiding". Now the word has strong connotations of a politician who panders to emotions and prejudice.

 •Egregious—Originally described something that was remarkably good. The word is from the Latin egregius "illustrious, select", literally, "standing out from the flock", which is from ex—"out of" + greg—(grex) "flock". Now it means something that is remarkably bad or flagrant.

•Guy—Guy Fawkes commemorated by parading and burning a ragged, grotesque effigy of Fawkes, known as a Guy. This led to the use of the word guy as a term for any "person of grotesque appearance" and then by the late 1800s—especially in America—for "any man", as in, e.g., "Some guy called for you." Over the 20th century, guy has replaced fellow in America, and, under the influence of American popular culture, has been gradually replacing fellow, bloke, chap and other such words throughout the rest of the English-speaking world. In the plural, it can refer to a mixture of genders (e.g., "Come on, you guys!" could be directed to a group of men and women).

•Gay—Originally meant (13th century) "lighthearted", "joyous" or (14th century) "bright and showy", it also came to mean "happy"; it acquired connotations of immorality as early as 1637, either sexual e.g., gay woman "prostitute", gay man "womanizer", gay house"brothel", or otherwise, e.g., gay dog "over-indulgent man" and gay deceiver "deceitful and lecherous". In America by 1897 the expression gay cat referred to a hobo, especially a younger hobo in the company of an older one; by 1935, it was used in prison slang for a homosexual boy; and by 1951 and clipped to gay, referred to homosexuals.