A woman with a smoking hot body but a rude head. Just deal with her like you would a prawn; keep the body throw away the head.
Dave hooked up last night, he nailed a prawnie
An excessively curved penis, such that it looks like a prawn.
Or, an excessively phallic prawn, such that it looks like a penis.
"Theres no way I'm putting that prawnis in my mouth."
His prawnis is always getting in the way.
Luke: "What are you getting the missus for christmas?"
Simon: "Vagisil, she's got a bad case of the prawny fanny"
Luke: "Yeah she does ay"