WordNet sense 1 (not as expected):

fishy, shady, suspect, suspicious, suspect

WordNet sense 2 (open to question):

equivocal

WordNet sense 3 (open to doubt or suspicion):

doubtful, dubious

WordNet sense 4 (open to doubt or debate):

debatable, problematic, problematical

WordNet sense 5 (being of questionable authenticity):

apocryphal

WordNet sense 6 (able to be refuted):

refutable, confutable, confutative

WordNet sense 7 (capable of being denied or contradicted):

deniable