WordWeb Online

1. Open to doubt or suspicion
   "the candidate's questionable past";
   - doubtful, dubious, dubitable, in question, doubtable
    
2. Subject to question
   "questionable motives"; "a questionable reputation"; "a fire of questionable origin"

See also: alleged, apocryphal, contestable, debatable, deniable, dodgy [Brit, informal], equivocal, fishy [informal], funny, impugnable, problematic, problematical, self-styled, shady [informal], so-called, soi-disant, supposed, suspect, suspicious

Antonym: unquestionable

Encyclopedia: Questionable