Green’s Dictionary of Slang

spitless adj.

In phrases

scared spitless (adj.) [euph. for scared shitless under scare v.]

absolutely terrified.

1927		(con. 1917) J. Stevens Mattock 232: You got him scared spitless already.
1945		R.L. Bellem ‘Dead Don’t Dream’ in Hollywood Detective July 🌐 You accused him of murder—which scared him spitless.
1979		S. King Dead Zone (1980) 117: He’s scared. Scared spitless.
1983		‘Heat Moon’ Blue Highways 27: I’d love to do that, but I’d be scared spitless.