1. slang A foolish, naïve, or gullible person. You're a bigger sap than I thought if you believe that, Phil. I can't believe I fell for that scam. I'm such a sap!

2. slang Someone who is easily taken in, won over, or entertained by something. Usually followed by "for (something)." A: "I know the film wasn't perfect, but I loved it." B: "Yeah, but you're a sap for romantic comedies." I've always been a sap for mysteries and thrillers with lots of twists and turns.

3. obsolete slang A short club or truncheon, especially as used by police. The officer pelted the man across the brow with his sap, knocking the poor fellow to the ground.