The standard expression is nip it in the bud, not nip it in the butt. The phrase comes from gardening. To nip means to pinch or cut off, and a bud is the part of a plant that has not opened yet.
Because of that image, the meaning is clear: stop something at an early stage, before it grows into a bigger problem. It is often used for trouble, conflict, bad habits, or rumors.
- Rumor: We should nip the rumor in the bud.
- Problem: The manager tried to nip the issue in the bud.
- Bad habit: Parents often want to nip rude behavior in the bud.
Nip it in the butt is a common mistaken version because it sounds similar, but it is not the original phrase. In careful writing, use nip it in the bud.