* sperm whale: yes, this can also exact puerile giggling from native speakers

* "You can't have your cake and eat it, too": might be better phrased as "can't eat your cake and have it, too". The idiom is stuck in its given form, however.

* blow job: I have read this etymologically arose from "below job". there are occasional humorous anecdotes related to native speakers misunderstanding it as well

* first off: never really thought about this before. I would suppose it comes from metaphorically popping the first of many arguments off of the speakers mental list.

* ladyfinger: it meant long like fingers, the lady part meant to indicate slender or delicate. there was a plant called ladyfingers as well