A rude insult for a person you find annoying and weak. You are not saying they are dangerous. You are saying they are small — like an ant — and that they have no backbone. People throw it at someone they refuse to respect: a piss-ant clerk who blocks you with a rule, a piss-ant rival, a piss-ant boss who nags. It is mainly American, very informal, and it is meant to shrink the person. Though actually, you will also see it written as one word, pissant; that is the same insult, not a different word. This probably began as a name for a kind of ant that people said smelled of urine, and later it stuck to humans as “tiny and not worth your time.” Keep it off a real person if you still want to be kind. In careful talk, pest, nobody, or small-time is enough.