knock (one's) brains out

1. To strike one with a violent blow to the head. A: "Oh man, what happened?" B: "You don't remember? Well, you and Bart were fighting, and then Bart knocked your brains out." I was doing work under the countertop, and I nearly knocked my brains out on it when I stood up!

2. To put forth one's maximum effort. I've been knocking my brains out trying to get a passing grade this semester, so failing by two measly points is incredibly frustrating. Don't knock your brains out trying to please these people—they'll never appreciate it.