incomprehensible — impossible to understand or comprehend; unintelligible.
indescribable — not describable; too extraordinary for description: a scene of indescribable confusion; indescribable euphoria.
unique — existing as the only one or as the sole example; single; solitary in type or characteristics: a unique copy of an ancient manuscript.
singular — extraordinary; remarkable; exceptional: a singular success.
uncommon — not common; unusual; rare: an uncommon word.
illogical — not logical; contrary to or disregardful of the rules of logic; unreasoning: an illogical reply.
unreasonable — not reasonable or rational; acting at variance with or contrary to reason; not guided by reason or sound judgment; irrational: an unreasonable person.
unimaginable — capable of being imagined or conceived.
incredible — so extraordinary as to seem impossible: incredible speed.
improbable — not probable; unlikely to be true or to happen: Rain is improbable tonight.
unbelievable — too dubious or improbable to be believed: an unbelievable excuse.
unheard-of — that was never heard of; unknown: an unheard-of artist.
strange — unusual, extraordinary, or curious; odd; queer: a strange remark to make.
fantastic — conceived or appearing as if conceived by an unrestrained imagination; odd and remarkable; bizarre; grotesque: fantastic rock formations; fantastic designs.
phony — not real or genuine; fake; counterfeit: a phony diamond.
rare — Réseaux Associés pour la Recherche Européenne
staggering — tending to stagger or overwhelm: a staggering amount of money required in the initial investment.
thin — having relatively little extent from one surface or side to the opposite; not thick: thin ice.
unconvincing — persuading or assuring by argument or evidence: They gave a convincing demonstration of the car's safety features.
unlikely — not likely to be or occur; improbable; marked by doubt.
unsubstantial — not substantial; having no foundation in fact; fanciful; insubstantial: an unsubstantial argument; unsubstantial hopes.
weak — not strong; liable to yield, break, or collapse under pressure or strain; fragile; frail: a weak fortress; a weak spot in armor.
unknowable — not knowable; incapable of being known or understood.
far-fetched — improbable; not naturally pertinent; being only remotely connected; forced; strained: He brought in a far-fetched example in an effort to prove his point.
fishy — like a fish in shape, smell, taste, or the like.
flimsy — without material strength or solidity: a flimsy fabric; a flimsy structure.
out of the question — a sentence in an interrogative form, addressed to someone in order to get information in reply.
outlandish — freakishly or grotesquely strange or odd, as appearance, dress, objects, ideas, or practices; bizarre: outlandish clothes; outlandish questions.
questionable — of doubtful propriety, honesty, morality, respectability, etc.: questionable activities; in questionable taste.
suspect — to believe to be guilty, false, counterfeit, undesirable, defective, bad, etc., with little or no proof: to suspect a person of murder.
untenable — incapable of being defended, as an argument, thesis, etc.; indefensible.
cockeyed — If you say that an idea or scheme is cockeyed, you mean that you think it is very unlikely to succeed.
