I felt like cooking up a reply because you might be the same as me - I thought the genders were flipped, and I couldn’t understand what the girl was saving up. Since it is a guy, and going by the fact that the OP made it look like it was something that “only guys would know” I’m assuming it has something to do with penis. Going off of that, the only thing that I can think of is “saving up cum” since, as far as I know, the longer a man abstains, the stronger his orgasms, and the more ejaculate is expelled?

I didn’t find it funny though. Either this is not my type of humour, either I misunderstood the joke.

Maybe I’m the same as you and my reply contributed nothing to the discussion ;)