blue balls

Blue balls is slang for an uncomfortable testicular sensation that can occur during a state of male sexual arousal. The term is thought to have originated in the United States, first appearing in 1916. Another slang term used for the condition is lover's nuts. Some urologists call this condition epididymal hypertension or sexual arousal orchialgia.
Most often it describes a temporary fluid congestion (vasocongestion) in the testicles, caused by prolonged sexual arousal in the human male without ejaculation.

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