1. To be experienced by or befall someone or something. I hope something bad didn't happen to Marie—she's never this late for a meeting. A: "What on earth happened to my book?" B: "Oh, my Great Dane sat on it and bent the cover—sorry."

2. To do something unintentionally; to experience something by chance. I just happened to see your mother today—what a lovely woman! If you happen to see my copy of The Catcher in the Rye, could you put it aside for me?