Green’s Dictionary of Slang

of a situation/person, suspicious, dubious.

1965		L.J. Cunliffe Having it Away xiv 97: It seemed a bit sussy to me.
1974		G.F. Newman You Flash Bastard 169: ‘Wouldn’t you be better off giving it a miss ? It’s a bit sussy, Terry.’ ‘You worry too much, son [...] Be all right.’ ‘You hungry bastard – you must’ve earnt more than enough. Drop him right out.’.
1990		J. Healy Streets Above Us (1991) 30: He tries to get rid of the overcoat because it makes him look too sussy.