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An immensely painful zit that develops seemingly under the epidermis and makes that spot sensitive to touch for days. A bit of a scientific oddity, since zits are supposed to be infected pores, but these things clearly have deeper intentions.
Jake: Whoa dude did you get hit in the forehead with a tack hammer?

Rob: What? Wait - sigh. It's my underground pimple.

Jake: Jesus dude, stick a needle in it already.
by Anchova June 11, 2011
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