1. assistance or backing that is uncertain, unreliable, or questionable in nature
The project received only doubtful support from the board members.
O projeto recebeu apenas apoio duvidoso dos membros do conselho.
2. help or endorsement given with hesitation or lack of confidence
Her doubtful support for the initiative suggested she had reservations.
Seu apoio duvidoso à iniciativa sugeriu que ela tinha ressalvas.