A characteristic tightness and or plumpness which a woman's (or man's) bum displays upon observation. This particular type of bum will instantly imprint an image of a tight, lubricated and hot bum in the viewers mind upon observation.
Jody: "Keith!, Keith are you listening!?"
Keith: "Sorry, I couldn't help thinking about your bum, it's a smiggle bum!"
*Jody slaps keith and walks away