impossible
inconceivable
out of the question
unimaginable
a bit thick
a bit thin
absurd
barred
beyond belief
closed-out
contrary to reason
doubtable
doubtful
dubious
dubitable
excluded
extraordinary
hard of belief
hard to believe
hopeless
illogical
implausible
improbable
incogitable
incomprehensible
incredible
insupposable
laughable
logically impossible
ludicrous
mind-blowing
mind-boggling
not deserving belief
not possible
open to doubt
open to suspicion
oxymoronic
paradoxical
passing belief
preposterous
problematic
prohibited
questionable
rare
ridiculous
ruled-out
self-contradictory
singular
staggering belief
suspect
suspicious
impracticable
inadmissible
inapplicable
incalculable
indefensible
repugnant
unable
unbelievable
unfathomable
unfounded
unheard-of
unintelligible
unpardonable
unspeakable
untenable
valueless
unsinkable
thinkable