The euphemisms are full of examples like that. Not only can they confuse you in the way you meant, you can just plain miss them. Just yesterday I learned that "watery knees", when used in the context of being very afraid, doesn't necessarily refer to what I thought it referred to, which was just shakey knees that were as stable as water. It refers to pissing oneself, hence, literally, "water" (itself a common historical euphemism for piss) actually on the knees, as well as everywhere else. I don't know how many times I've seen that without putting that together.

(Pardon the bluntness, but explaining one euphemism with another seems rather pointless.)