Professor John McWhorter, an author of linguistic works and other books who has written for many top magazines and newspapers and has appeared on a host of TV shows ranging from the Jim Lehrer NewsHour, to Today and Good Morning America, to Politically Incorrect, likes to illustrate the drift of the meaning of words (in all languages) this way:
In the 1940s, Jack Benny Radio Show band leader Phil Harris, a randy sort, asked his wife to agree that "Nobody makes love better than me." Rather risqué for a 1940s national radio show, wouldn't you say? Of course, Harris still used the term "making love" in a sense that had to with wooing a woman. No listener thought of it in the modern sense.
We're all used to slang expressions changing, and usually dropping out of sight. When was the last time you heard somebody in the real world talk about "hep cats?" (Think members of the groovy "beat generation," dressed in black and snapping their fingers to a poetry reading in some basement club in New York.)
But change isn't limited to slang. The meanings of all words change. All the time.
In English, the classic example is the word "silly." It means foolish, right? Back in old English, it meant "blessed." (So did its cognate in German.) But blessed can also mean "innocent," which "silly" came to mean. Years later "silly" made another natural progression to meaning "deserving of compassion." (That's the meaning that Shakespeare used in 1591 in a line from The Two Gentlemen of Verona where a character warns another not to do outrages "On silly woman or poor passengers." He didn't mean not to hit on air-head blondes, he meant to show compassion to women as well to the poor.) Eventually, the meaning of silly changed again. After all, those deserving compassion were "weak," and that's what the word came to mean. From there, the connotation of "simple" or "ignorant" was just another natural step.
I've used the word "wicked" to show what may be a meaning change that we are living through right now: The move from "evil" to "really cool." But that raises another question. Using the word "cool" to mean "really groovy" has had a good run, but how long can it last?