Blue balls is a slang term for a medical condition called epididymal hypertension. This condition occurs when prolonged sexual arousal without ejaculation causes fluid to build up in the testicles.

The next time someone uses blue balls as an excuse, you’ll know the truth. Are blue balls real? [LloydsPharmacy] You may have heard people joke about "blue balls" or use the term to explain discomfort ...

If you are a sexually-active heterosexual woman, then you had had a man accuse you of giving him “blue balls” when you aroused him, but then would not engage in sexual activity with him. He did so ...