Definition of "improbable"
adjective
comparative more improbable, superlative most improbable
Not likely to be true.
Quotations
Nay, if he be of a proud humour, […] he will not Bate an Ace of abſolute certainty, but however doubtful or improbable the thing is, coming f[r]om him it muſt go for an indiſputable truth.
1674, [Richard Allestree], “Of Positiveness”, in The Government of the Tongue. […], Oxford, Oxfordshire: At the Theater, page 197