Longman Dictionary of Contemporary English
Douglas Harper's Etymology Dictionary
1926, from piss + -y (2). Figurative use by 1972.
Wiktionary
a. 1 Covered in piss. 2 Resembling or smelling like piss. 3 anal retentive, whinge and pernickety. 4 (context of rain English) weak and drizzly.
Wikipedia
Pissy is a commune in the Somme department in Picardie in northern France.
Usage examples of "pissy".
Within a month, local chauffeurs, cooks and houseboys had swelled the ranks and Pissy Johnson, Cunning-Spider and Atherton, as well as two guys from School House who could speak Sotho, were roped in to teach on Saturday nights.
I probably picked it up from my sister, Bethany, who was fourteen at the time and spent hours in front of the mirror rolling her eyes and practicing pissy looks to advertise her so-called angst.
Well like I always said, someday somebody's going to kill Bellbottom just to flay him and use his skin for a convertible roof, but Dogwalker just waved and walked on by while I made little pissy bumps at Bell.
Rarely, when there was no meat displayed on the low-price counter, the pot was filled with innards: knotted fatty beef hearts, pissy, because unsoaked, pig's kidneys, also small lamb kidneys which my mother had to detach from a finger-thick coat of fat lined with crackling parchment: the kidneys went into the dog pot, the suet was rendered in a cast-iron frying pan and used in the family cooking, because mutton suet wards off tuberculosis.