WordWeb Online

1. Subject to question
   "questionable motives"; "a questionable reputation"; "a fire of questionable origin"
    
2. Able to be refuted
   - refutable, confutable, confutative

See also: alleged, apocryphal, contestable, debatable, deniable, doubtable, doubtful, dubious, dubitable, equivocal, fishy, funny, impugnable, in question, problematic, problematical, questionably, self-styled, shady, so-called, soi-disant, supposed, suspect, suspicious

Antonym: unquestionable

Encyclopedia: Questionable