the appearance of the testicles after anal sex when any 'bum gravy' (a bit like fanny batter but brown in colour and from the anus) residue has been allowed to dry, thus making it look like you have just dipped your sweaty bollocks in the Bisto.
Eee and Col were having a threesome again and lucky Col got the chance to 'smash her backdoors in'. After some frantic thrusting he eventually finished and withdrew only to find his privates covered in bum gravy. Too tired to wipe his knob he fell asleep and allowed the bum gravy to dry. When he and Eee eventually woke up Eee remarked 'love the look of the Bisto bollocks mate'.