(a.) Not probable; unlikely to be true; not to be expected under the circumstances or in the usual course of events; as, an improbable story or event.

absurd crazy distant doubtable doubtful dubious dubitable extraordinary fanciful farfetched fishy flimsy forced hard to believe hardly possible implausible impossible inconceivable incredible insane ludicrous mad more than doubtful more than expected out-of-the-way past expectation peculiar questionable quite another thing remote ridiculous something else again strained strange sudden thin unannounced unanticipated unbelievable unconvincing undivined unexpected unforeseeable unforeseen unguessed unheralded unhoped for unlikely unpredictable unpredicted unprepared for unpromising unrealistic unsubstantial unthinkable weak weird wild without warning