Blue balls is the pop-cult-approved term for the uncomfortable sensation that men feel when they're aroused but unable to, shall we say, see it to the finish line.
While not a medical term, "blue balls" certainly seems to capture a male's overall disappointment with not getting any nooky as well as the discomfort and pressure they feel in their testicles. It's not a figment of imagination nor a pressure tactic and it can be even more unpleasant for those who have been aroused for a while or on multiple occasions, without the gratification of release...
The term actually describes a very real bodily function that occurs. When they're aroused and know the possibility of sex is on the horizon, men's bodies will respond by getting everything ready. Blood rushes genital area, swelling the penis and making it erect, ready for penetration.
The flow and pressure of the blood is the body's way of signaling to the brain the promise of sexual pleasure to come - which is felt as a quickening of pulse and excitement or arousal, dilating pupils and a variety of other triggers.
If, however, a man can't actually indulge in the activity his entire body is clearly preparing for, then they experience a very slow and somewhat uncomfortable "deceleration". Blood leaves the area, the genitals relax and "deflate", which can be a strange sensation, especially when coupled with the sense of mental frustration at not being able to complete.
While it certainly is annoying and causes men to cheekily compare their discomfort to what a woman must feel during her period, rest assured it's not harmful (and nor is the latter comparison necessarily true).
A good remedy for blue balls is to have a nice shower jerk-off session or masturbate in bed in the morning. Getting those juices flowing, so to speak, can really put a spring in your step. The alternative is waiting it out...which is sometimes not feasible for those of us who aren't monks or practicing tantric sex.
In popular culture, it's common to conflate a man experiencing blue balls as less of a man or one who just can't "win" with the ladies. Or the "discomfort" is exaggerated for comic effect. But this is not an accurate representation of the experience itself.
There are many logistical reasons why a man might experience this: a long distance relationship or a busy work schedule that cuts into intimate time. Often, even single guys who are not looking for girlfriends can experience "blue balls" because arousal can happen at any time and the penis and scrotum are very sensitive areas.
And ladies, blue balls is not a pressure tactic and you should never allow a guy to use it as such!