Butt nipper

I've written before about my amazing misunderstandings of common phrases, like when my roommate told me she was doing P90X, and I tried to talk her out of doing drugs. Or when I thought  prima dona was "pre-Madonna". But most recently I have been using the term "nip it in the bud" wrong for years, and NO ONE HAS CORRECTED ME. Until now. I always thought this was nip it in the BUTT. Vastly different it seems. Who knew? As a side note, I refuse to google "butt nipper" for the sake of an accompaniment, but by all means, go right ahead.