A run-in is a brief, unexpected conflict or confrontation with someone. When you have a run-in with the school bully in the hallway, you're not looking for trouble, but suddenly there's an unpleasant encounter. Your older sister might have a run-in with a difficult customer at her job, or your dad might tell a story about a run-in he once had with an aggressive dog while jogging.

The phrase suggests something that happens quickly and catches you off guard. It's not a long, planned argument or a major fight, just a tense moment that flares up and usually ends fairly quickly. You might say, “I had a run-in with my teacher about my late homework,” meaning there was a brief, uncomfortable confrontation about it.

Most run-ins are minor and forgettable, though some can create tension that lasts. The word almost always describes negative encounters. You wouldn't say you had a run-in with someone who complimented your science project.