Any of a number of ants that can be found indoors. So-named because they make your fingers smell like micturition (piss) when you squish them. The odour comes primarily from formic acid in the insect's body that is released when you kill it by smashing it between your fingers or squish it between your finger and a wall, sink, floor, etc.
I just killed threedozen piss ants in my bathroom lavatory! Now my fingers stink like piss!!! :-O