Green’s Dictionary of Slang

1. (US campus) acceptable, satisfactory [for ety. see cas adj.].

1959		L.P. Boone ‘Gator Sl.’ AS XXXIV:2 156: Crazy said red, and it’s casual are comparative newcomers to this list of superlatives that mean whatever the speaker wants them to mean.
1977		Eble Campus Sl. Apr. 1: casual – acceptable, good, fun: I thought I’d study a little then go out with Bill. That’s casual!
1984		Eble Campus Sl. Sept.

2. not worth becoming upset about.

1984

C. Crowe Wild Life [film script] ‘Sorry.’ ‘That’s casual.’ [Ibid.] ‘I’ve been doing your job for two hours. Get to work!’ ‘It’s casual.’.

3. pertaining to football hooliganism.

1998

I. Welsh Filth 80: A seasoned casual moll if ever there was one.