The story
From World War II military slang. Construction obscure; suggestions are: * From come (“act the part of”) + the + raw (“naive”) + prawn (“(slang) fool”) — thus, to attempt to deceive by feigning ignorance. * From come (“(putatively) perpetrate”) + the + raw prawn = something hard to swallow.
Etymology from Wiktionary, CC BY-SA 4.0.