unthinkable (adj.)

• absurd
• barred
• doubtable
• doubtful
• dubious
• dubitable
• extraordinary
• hopeless
• illogical
• implausible
• impossible
• improbable
• incomprehensible
• inconceivable
• incredible
• laughable
• ludicrous
• mind-blowing
• mind-boggling
• paradoxical
• preposterous
• problematic
• prohibited
• questionable
• rare
• ridiculous
• self-contradictory
• singular
• suspect
• suspicious
• tall
• thick
• thin
• unacceptable
• unbelievable
• uncommon
• unconvincing
• unearthly
• ungodly
• unimaginable
• unique
• unlikely
• unusual
• unwonted