Choose the adjective dubious for something you have doubts about or you suspect is not true. That bridge you just "bought" might be of dubious value.
Dubious stems from Latin dubiosus "doubtful" or "uncertain" and contains the Latin root duo in this case meaning "of two minds." This is apparent in that dubious generally describes something that appears one way but is truly another. A dubious claim is probably not true, whereas a dubious website or character is of questionable quality. Dubious can also be synonymous with doubtful when referring to a person as in "she was dubious about the idea."