"urine," late 14c., from piss (v.). As a pure intensifier (piss-poor, piss-ugly, etc.) it dates from 1940, popularized in World War II. Piss and vinegar "vim, energy" is attested from 1942. Piss-prophet "one who diagnoses diseases by inspection of urine" is attested from 1620s. Piss proud "erect upon awakening" is attested from 1796.
adjective suffix, "full of or characterized by," from Old English -ig, from Proto-Germanic *-iga- (source also of Dutch, Danish, German -ig, Gothic -egs), from PIE -(i)ko-, adjectival suffix, cognate with elements in Greek -ikos, Latin -icus (see -ic). Originally added to nouns in Old English; used from 13c. with verbs, and by 15c. even with other adjectives (for example crispy).
<a href="https://www.etymonline.com/word/pissy">Etymology of pissy by etymonline</a>
Harper, D. (n.d.). Etymology of pissy. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/pissy