Green’s Dictionary of Slang

drap(e) sack n.

(Aus.) a fool.

1950		Dly Teleg. (Sydney) 31 Dec. 7/3: Melbourne saxophone player Ross Fusedale said ‘bodgies’ and ‘long hairs’ were barred from jazz. ‘There's no long hairs or drape sacks here,’ he said. ‘Jazz is a serious art form’.
1993		Aus. Word Map 🌐 drap sack. A fool or a boofhead.