Green’s Dictionary of Slang

1. an unfair action or circumstance, anything far-fetched; thus come the raw prawn, to act resentfully or unpleasantly; to be rude.

1957		‘Nino Culotta’ They’re a Weird Mob (1958) 85: Drawn the raw prawn again.
1965		J. O’Grady Aussie Eng. (1966) 74: Aw, don’t come the raw prawn, mate—go an’ find another mug.
1983		J. Byrell (con. 1959) Up the Cross 115: Brucie [...] copped the don’t-come-the-raw-prawn treatment. So he fastened his yap.

2. someone who is easy to deceive, a dupe.

1959		(con. 1940s) E. Lambert Glory Thrown In (1961) 29: Don’t ever come the raw prawn with Doc, mate. He knows all the lurks.
1968		S. Gore Holy Smoke 47: I dunno who’s trying to hand who the raw prawn.
1970		Galton & Simpson ‘Cuckoo in the Nest’ Steptoe and Son [TV script] Well, don’t let’s stand here like a couple of raw prawns.
2001		N. Barlay Hooky Gear 162: Brawn an brain need raw prawn for dem game. An here I am all use up an raw like a fuckin prawn in a game.