Name a word that doesn't mean what most people think it means

*Effete *- means lacking in vigour, exhausted, sterile. The “effeminate” meaning is a modern nuisance.

Noisome - means disgusting or harmful (typically, as an odour). Not loud.
My favourite word of the moment, however, is pleonasm. It pretty much means redundant. It’s just that we already have a word for redundant…