To be associated with the word "timmy two pumps" is not the most flattering of things. Simply put, it means you can't hold your load. Even simpler put - it means you're a premature ejaculator.
Dude1: bro! Why aren't you in the other room with that girl anymore?
Dude2: Man, I took off my pants and splooged everywhere. It was over before it started. She then proceeded to call me Timmy Two Pumps.
Dude1: godamnit, dude.