Blue balls are when you become sexually aroused but don't orgasm/ejaculate, causing discomfort.
A more technical explanation:


" "Blue balls" is a slang term referring to testicular aching that may occur when the blood that fills the vessels in a male's genital area during sexual arousal is not dissipated by orgasm.


When a man becomes sexually excited, the arteries carrying blood to the genital area enlarge, while the veins carrying blood from the genital area are more constricted than in the non-aroused state.


This uneven blood flow causes an increase in volume of blood trapped in the genitals and contributes to the penis becoming erect and the testicles becoming engorged with blood. During this process of vasocongestion the testicles increase in size 25-50 percent.


If the male reaches orgasm and ejaculates, the arteries and veins return to their normal size, the volume of blood in the genitals is reduced and the penis and testicles return to their usual size rather quickly.


If ejaculation does not occur there may be a lingering sensation of heaviness, aching, or discomfort in the testicles due to the continued vasocongestion. This unpleasant feeling has popularly been called blue balls, perhaps because of the bluish tint that appears when blood engorges the vessels in the testicles.


The condition usually does not last long and the level of pain associated with blue balls is usually minor and can be exaggerated. Most men have been socialized to ejaculate when they get an erection during sexual activity. Failure to ejaculate and to feel orgasm often adds frustration and disappointment to the reality of the physical sensation.

Men who believe that they should ejaculate every time they have an erection are likely to exert pressure on their partner to proceed with sex without taking her feelings into consideration.
Some men find that masturbation is a viable solution and are realizing that ejaculation is not a requirement in every sexual situation.

This attitude allows both men and their partners to relax more and to learn that pleasure and meaning can exist without having to reach ejaculation and orgasm during every sexual encounter.


Men are not alone in experiencing the discomfort of unrelieved vasocongestion. Women's genitals also become engorged with blood during sexual arousal and, like their male counterparts, women can experience pelvic heaviness and aching if they do not reach orgasm."

cc: lalasticlala

So does priaprism which is what happens when the p.enis gets stuck in your partner.