I'm always fascinated by language.
Take the word gay. Nowadays it has two meanings in general use. One is for people of a homosexual persuasion, the other is used where something is a bit rubbish - as in his clothes were gay.
But think back a bit.
Not so many years ago, a gay person was described as a queer. And you might reasonably say you were feeling a bit queer, or your typewriter was acting queer, where the meaning was that you were feeling rubbish and your typewriter was rubbish, or strange.
So the meaning of gay has taken the same sense as the original term for, well, queer .