questionable

adjectives

subject to question

"questionable motives"; "a questionable reputation"; "a fire of questionable origin"

questionable

antonym

unquestionable

is similar to

doubtful or suspect

"these so-called experts are no help"

alleged, so-called, supposed

is similar to

being of questionable authenticity

apocryphal

is similar to

open to doubt or debate

"If you ever get married, which seems to be extremely problematic"

debatable, problematic, problematical

is similar to

open to doubt or suspicion

"the candidate's doubtful past"; "he has a dubious record indeed"; "what one found uncertain the other found dubious or downright false"; "it was more than dubitable whether the friend was as influential as she thought"- Karen Horney

doubtful, dubious, dubitable, in question

is similar to

open to question

"aliens of equivocal loyalty"; "his conscience reproached him with the equivocal character of the union into which he had forced his son"-Anna Jameson

equivocal

is similar to

not as expected

"there was something fishy about the accident"; "up to some funny business"; "some definitely queer goings-on"; "a shady deal"; "her motives were suspect"; "suspicious behavior"

fishy, funny, shady, suspect, suspicious

is similar to

subject to being discredited

impugnable

is similar to

as claimed by and for yourself often without justification

"the self-styled `doctor' has no degree of any kind"

self-styled, soi-disant