The origin of the term piss ant/pissant (and the archaic, English country dialect variant, pismire) is to be believed in reference to members of the Formica rufa group which bend their gasters forward and produce a visible stream of formic acid when their nest is disturbed.

Nowadays, since most people are not familiar with F. rufa and relatives (All european species are threatened and have legal protection, and most of the American ones probably ought to also.), the term is used as a general and usually derogatory term for any small ant. In the rural speech of southeastern USA, the belittling "piss ant"/"pissant" contrasts with the respectful (or perhaps fearful is more accurate) expression "bull ant", used for large ants such as Pogonomyrmex badius, Camponotus and even Odontomachus. --Dr. Ant