Sometimes the source of a mutation can be hard to pinpoint. Take, for example, the word nimrod. That word has always denoted a hunter. It derives from a name in Genesis: Nimrod, a descendant of Ham, was a mighty huntsman and king of Shinar. Most modern dictionaries even capitalize the English word. But few people today capitalize Nimrod, and fewer still use it to mean "great hunter." The word has depreciated in meaning: it's now pejorative, denoting a simpleton, a goofy person, a dummy.
— Garner's Modern English Usage
by Bryan Garner