To buttonhole someone means to stop them and hold their attention when they're trying to leave or do something else. Imagine you're heading out the door when someone grabs you by the sleeve (or figuratively, by your buttonhole) and starts talking, making it hard to escape politely.

You might see this at a school event when an enthusiastic parent buttonholes the principal to discuss every detail of an idea, or when a talkative neighbor buttonholes your dad in the driveway and he can't get away to mow the lawn. The person doing the buttonholing usually doesn't realize they're trapping someone who wants to leave.

The word comes from an old practice of literally holding onto someone's buttonhole (the slit in clothing where a button goes through) while talking to them, which made it nearly impossible to walk away without being rude. Today we don't actually grab people's clothes, but the feeling is the same: you're stuck listening when you'd rather be somewhere else.

A buttonhole is also, of course, simply the slit in clothing that a button passes through to fasten it.