Green’s Dictionary of Slang

coup de n.

(Aus. prison) excellent, first-rate.

[Aus]‘Siriusly’ Speaking in Just Us (H.M. Prison, Beechworth, Vic.) Dec. 25/2: coup de 1. a. Grouse, terrific; peachy (what a coup de day) 2. adv. (that’s coup de) [from coup de grâs [sic].