Which is correct?
You can imagine a use for “nip it in the butt” — you could deter someone that way but it doesn’t seem like office-appropriate behavior.
The actual saying is “nip it in the bud.” The meaning is to take care of a problem in its early stages before it gets too big to manage easily.
Here’s Paul Brians on the question:
“To nip a process in the bud is to stop it from flowering completely. The hilariously mistaken ‘nip it in the butt’ suggests stimulation to action rather than stopping it.”
It’s surprising how often “nip it in the butt” appears, especially online. I think this is yet another case of people mishearing something and not seeing it in print (because they don’t read enough, I’m thinking).
There are lots of examples of misuse, even within the media, especially the sports media (just saying):
“These were two crucial points for us,” said Bailey, who has four goals and nine points during a six-game point streak. “If you don’t get them, they’ll nip you in the butt at the end of the year.”
“… but I realized that if something starts to not feel good you’ve got to nip it in the butt quickly and not just keep going and get into a bigger problem …“
“Hopefully both sides will realize they caused this mess by letting people who are litigation experts lead negotiations, and next time, when this happens again, they’ll nip it in the butt before it causes a problem.”
“If the people around you are negative thinkers, it’s like having wolves at the door. It becomes an emotionally contagious atmosphere. Managers have to nip it in the butt. Once a company is in freefall, it’s too late.”
Then, again, the expression is occasionally used to good effect, whether or not the writer knows the correct expression:
“Something dangerous is going on in the world of women’s underwear, and I want to nip it in the butt. Sorry. I am referring, of course, to Spanx. If you don’t know what Spanx are, I have one word for you: Girdles.”
So there you have it. If you want to use “nip it in the butt” for a bit of wordplay, I have no quarrel with you. But if you’re just writing about stopping a small problem before it gets out of hand, save your dental work — the correct expression is “nip it in the bud.”