1. With the belief or expectation that one's actions will bring good fortune. A: "Because our team won today, we all have to wear the same clothes to next week's game for luck!" B: "Don't you think you're being a little superstitious?"

2. Used to describe something done for no apparent reason. Similar to the phrase "just because." Well, little brother, there's your 12 birthday punches—oh, and one more for luck! I was so exhausted after running all those laps that I nearly collapsed when the coach said "and one for luck!"