Blue balls

Blue balls is a slang term used by guys when they experience uncomfortable sensitivity, soreness, and aching in the testicles due to not having an orgasm. When a guy gets aroused, some blood starts to flow into the testicles and cause them to swell slightly and become a bit tender. If a guy keeps himself aroused for a long time without reaching orgasm and then stops or "backs off" from sex, the blood won't drain from the testicles and the guy will have blue balls.