adjective

hesitating or doubting.

I was rather dubious about the whole idea

doubtful,uncertain,unsure,in doubt,hesitant,undecided,unsettled,unconfirmed,undetermined,indefinite,unresolved,up in the air,wavering,vacillating,irresolute,in a quandary,in a dilemma,on the horns of a dilemma,sceptical,suspicious,iffy