Green’s Dictionary of Slang

(W.I., Jam.) a trickster, a confidence man.

1971		Cassidy Jamaica Talk (2nd edn) 215: There is a common phrase, ‘Chief in town, jinnal mus’ dive,’ chief meaning a gull or potential victim.
1978		‘Jamaica Child’ in P. Ashton et al. Our Lives (1982) 29: So no try fe trick me, cause me is a bigger jinnal dan you.
1980		M. Thelwell Harder They Come 198: Me say wake up Ramgoat, you damn Sunday ginnal you.
1995		Francis-Jackson Official Dancehall Dict. 22: Ginnal (Ja.) a trickster; con-man: u. ’im a big time ginnal.
2001		(con. 1981) A. Wheatle East of Acre Lane 8: One pound fifty fe come in, especially for you. I don’t like ginall.
2015		410 ‘Civilians’ 🎵 Just catch me a ginal.