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.