A slang term commonly used amongst Bodybuilders to refer to the feeling in which blood is 'pumped' into the muscle for support during overload causing it to expand in size and becoming tight, thus being 'pumped' up.
Famously compared to cumming by Arnold Schwarzenegger in the classic Bodybuilding documentry "Pumping Iron".
The most satifying feeling you can get in the gym is the pump. Let's say you train your biceps, blood is rushing in to your muscles and that's what we call the pump. Your muscles get a really tight feeling like your skin is going to explode any minute and its really tight and its like someone is blowing air into your muscle and it just blows up and it feels different, it feels fantastic. Its as satifying to me as cumming is, you know, as in having sex with a woman and cumming.