I'm guessing "Fritz" is a slang term for the Germans. "Hit them (Hit'um) where it hurts (smarts)" means to do damage with blows or bullets or whatever, and "ground him" could be either shooting an airplane down or forcing the other air force to stay on the ground to avoid damage.
New words, one handy idiom, and a 2-minute quiz — delivered to your inbox to keep your streak alive.
1caitlyn1If we keep hitting “Fritz” where it smarts, eventually we’ll ground him.If we keep striking at the Germans where they are most vulnerable, eventually we will win the air battle against them.