Green’s Dictionary of Slang

shit adv.

[1930s+] as an intensifier extremely, very, completely, certainly.

In compounds

shitsure (adv.)

[1950s+] (US) certainly, definitely; also as adj., certain.

In phrases

shit out of luck (also shit out)

[20C+] (US) at the end of one’s good fortune, in serious trouble with no escape.