Green’s Dictionary of Slang

red ass n.

also r.a., red-arse

1. [1940s] (N.Z.) an incompetent person.

2. [1940s+] bad temper, irritation.

3. [1970s] a scolding.

In derivatives

red-assed (adj.)

[1960s] (US) very angry.

In phrases

get the red ass (v.)

[1950s+] (orig. US) to bear a grievance, to be in a bad temper.

give the red ass (v.)

[1960s+] to infuriate.