This word has an interesting
etymology (and
entomology too,
come to think of it). It's related to the words
piss and
ant, but not in the way you might think.
Pismire is an
Old English word for "ant," evidently from the
Danish "myre," meaning "ant." The "pis" component is thought to come from the fact that many ants secrete
formic acid (it's what puts the
fire in
fire ants, for example). Formic acid, in high concentrations (such as
anthills) smells like
urine. So, pismire / pissant is a word that means "ant, which by the way smells like piss." One can see why it's an
insult that means about the same thing as calling someone an ant, but with an extra
edge.
source: The American Heritage® Dictionary of the English Language, Fourth Edition
Available at http://dictionary.com