Green’s Dictionary of Slang

flags n.

(Aus. teen) nickname of the favoured style of trousers among 1960s sharpie n.2 : ‘22" bottoms, usu. pinstripe, herringbone or Prince of Wales check, in various hues of brown and grey’T. Taylor p.17/1 .

[Aus](con. 1960s-70s) T. Taylor Top Fellas 44/2: We didn’t wear flags, although I had a pair, we mostly wore jeans.