adj. 25B6; adjective
I was doubtful about going : IRRESOLUTE, hesitant, vacillating, dithering, wavering, in doubt, unsure, uncertain, in two minds, shilly-shallying, undecided, in a quandary/dilemma, blowing hot and cold.
it is doubtful whether he will come : IN DOUBT, uncertain, open to question, unsure, unconfirmed, not definite, unknown, undecided, unresolved, debatable, in the balance, up in the air; informal iffy.
the whole trip is looking rather doubtful : UNLIKELY, improbable, dubious, impossible.
they are doubtful of the methods used : DISTRUSTFUL, mistrustful, suspicious, wary, chary, leery, apprehensive; sceptical, unsure, ambivalent, dubious, cynical.
this decision is of doubtful validity : QUESTIONABLE, arguable, debatable, controversial, contentious; informal iffy; Brit. informal dodgy.
confident, certain, probable, trusting.