Green’s Dictionary of Slang

pissless adj.

In phrases

scare pissless (v.)

to terrify.

1930		(con. 1915) ‘W.W. Windstaff’ ‘A Flier’s War’ in Longstreet Canvas Falcons (1970) 268: John and I were scared pissless by the speed [...] of those small wire and canvas trainers.
1932		(con. 1917–19) Dos Passos Nineteen Nineteen in USA (1966) 479: Turns out the sonofabitch was a dick. S––t I was scared pissless.