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A tattoo on the lower back thought to identify a woman as sexually promiscuous. Such a tattoo is a hypothetical target for semen should a man be having unprotected sex with a partner doggy style and decide to pull out just prior to ejaculation, hence cum coaster.

See: Tramp Stamp
"I pulled out and shot my load all over that tramp's cum coaster."
by Josepolis January 16, 2008
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