1. To react to something with strong emotion. The reaction can be to something positive or negative. I had to talk to Mr. Myers about the botched report today, and boy, did he flip his lid. I knew Aunt June would be excited to hear I'm getting married, but she totally flipped her lid!

2. To go insane. Used humorously. Everyone thought I'd flipped my lid when I quit my lucrative marketing job and moved to India.