Tattoo on the small of a woman's back, usually roundish with rays or flames and two colors (green and/or red will be invovled most often), located precisely where a guy should set his beer while nailing her doggie style. Most prevalent with porn stars, wannabe porn stars, biker chicks, drrrty grrrls, and suburban fluff chicks who want everybody to think they're hard.
Gents - Are you horny? Then hit the club and scout for chicks with the Bar Slut Hat Trick - crop top, low-rider pants or skirt, and beer coaster tatt on display. (Tight jeans with high heels also a good indicator.)