Green’s Dictionary of Slang

dogfish n.

(US) a confidence trickster, a parasite.

H.B. Darrach Jr. ‘Sticktown Nocturne’ in Baltimore Sun (MD) 12 Aug. A-1/1: A dogfish is a shark that walks like a man; it is a point of honor among dogfish never to refuse a seaman ‘action,’ at a price.