Insertion of the thumb into the anus with the other hand raised above the head in an over-arching style such as an ape might perform when walking upright. The trousers and underwear should be at least at half-mast to allow the insertion of the opposing digit, or preferably you should be completely naked, to make it a classic Mogilla Gorilla. Use of the "oo-oo-oo" sound adds greatly to the effect.
The girls at the party were really prudish, so I dropped the Magilla Gorilla right through the middle of the lounge room. That livened things up!
A wet spot seen on the exterior of a man's trousers after he has urinated but not adequately shaken or wiped his penis, leading to an after-pee soiling of the nearby clothing. The size of the spot usually being reminiscent of a two-shilling, or "two-bob" coin used in pre-decimalised Australia and UK.
Hey Knackers, did you forget to give your cock a good shake after your snakes hiss? You've got a cracking two-bob spot on the front of your dacks there.