1. Dazed, lightheaded, or reeling from some blow to the face or head; punch-drunk. I could tell he was a bit slap-happy after the fight, as he kept calling me Martin for the rest of the evening. I'm sorry, I think I need to sit down for a bit. That collision has left me feeling a little slap-happy.

2. By extension, giddily or cheerfully happy, carefree, or inattentive. The children are so hopped-up on sugar and excitement from the party that they're all totally slap-happy and out of control. We'd been up so long working on the project that we were all getting a little slap-happy by the time we were finished.