Cudgel your brains
Definition 1 of 1
Pro Tip 1/3Intensity of Effort
The phrase implies using significant mental effort to remember or solve something. 
He had to cudgel his brains to solve the complex puzzle.
Pro Tip 2/3Idiomatic Expression
It's an idiom, so don't take 'cudgel' literally; it's about mental effort, not physical. 
I had to cudgel my brains to recall his name.
Pro Tip 3/3Slightly Informal
Mostly used in informal conversations rather than formal writing. 
Can you help me? I've been cudgelling my brains all day.