To hit somebody in the testicles while off guard, but only during a time of competitive game play with a very close friend. Both these are crucial points in classifying nut shots as clooping; this must be an ongoing war/competition between two friends trying to find the right moment when to move forward with a spontaneous attack to the nuts.
Friend A gets home from work, and greets Friend B outside on the Cloopfront porch, as they both continue to smoke their cigarettes. Once Friend B finishes his smoke, he turns around to walk back into their house. Friend A reaches down,and with a striking vertical blow/karate chop, racks Friend B in the balls screaming "Cloop bitch! You've been clooped"
Friend A walks by friend B and back hand slaps friend B in the nuts causing him to fall to his knees in a terribly uncomfortable pain. While laughing Friend C asks friend A what they're doing. "Clooping" Friend A calmly replies with a smirk on his face.
Cloop, Clooped, Clooping circa 2011