1. One is experiencing or will experience a negative outcome, particularly one that was intended for someone else. The phrase typically refers to a practical joke or malicious act that has backfired. Greg set up this bucket of water to fall on me, but the laugh's on him now—I rigged it up over his bedroom door. He thought he could steal my job by concocting terrible stories about me to tell the boss, but the laugh's on him—he's getting fired for misuse of company time.

2. One seems silly or foolish. Well, the laugh's on me, because I spent so much time perfecting our itinerary that I forgot to put gas in my car before the trip. I don't mind his tirades at all, actually, because the laugh is on him—Smith is our president whether he likes it or not.