Improbable means very unlikely to be true or to happen. A highly improbable story, an improbable victory, improbable but possible.
It leaves the door open where impossible closes it, which is the whole difference and the basis of Holmes's line about whatever remains. Improbable events happen constantly, since there are so many chances for something unlikely to occur to somebody.
1adjectiveimprobableVery unlikely to be true or to happen.
“It seems highly improbable that a letter would take ten years to arrive.”
improbable is a likelihood.
Latin improbabilis, im-, not, plus probabilis, provable or likely, from probare, to test.
Latin
improbābilis
English, today
improbable
Borrowed from Latin improbābilis. by surface analysis, im- + probable.
Etymology from Wiktionary, CC BY-SA