(transitive verb) To obscenely expose one's testicle or scrotum to another, either by pulling up one's shorts or by other means. A nut-flash is typically meant to frustrate or surprise the victim.
(noun) The action of nut-flashing.
Spencer: (To Lauren, aggravated) Laurrennn! Cooper nut-flashed me!
Cooper: No I didn't! I was only itching myself!