Idiom: to study one’s brains out
Jim: How did you do on the grammar test?
Sam: I got 100%. I studied my brains out for that test.
Jim: Nice job!
Chrissy: Are you preparing for the GRE exam?
Tammy: Yeah, I’ve been studying my brains out for the last 6 months.
Chrissy: Well I’m sure you’ll do great!
Meaning: to study one’s brains out means to study extremely hard for something.