adj. 25B6; adjective
I was rather dubious about the idea : DOUBTFUL, uncertain, unsure, hesitant; undecided, indefinite, unresolved, up in the air; vacillating, irresolute; sceptical, suspicious; informal iffy.
a dubious businessman : SUSPICIOUS, suspect, untrustworthy, unreliable, questionable; informal shady, fishy; Brit. informal dodgy.
certain, trustworthy.