improbable (adj.)

late 14c., "likely, reasonable, plausible, having more evidence for than against," from Old French probable "provable, demonstrable" (14c.), from Latin probabilis "worthy of approval, pleasing, agreeable, acceptable; provable, that may be assumed to be believed, credible," from probare "to try, to test" (see prove). As a legal term, probable cause "reasonable cause or grounds" is attested from 1670s.

Probable cause (used with reference to criminal prosecutions), such a state of facts and circumstances as would lead a man of ordinary caution and prudence, acting conscientiously, impartially, reasonably, and without prejudice, upon the facts within his knowledge, to believe that the person accused is guilty. [Century Dictionary]

Related: Probableness.

1590s, "fact or quality of being improbable;" see improbable + -ity. Meaning "an instance of something improbable" is from 1610s.

word-forming element meaning "not, opposite of, without" (also im-, il-, ir- by assimilation of -n- with following consonant, a tendency which began in later Latin), from Latin in- "not," cognate with Greek an-, Old English un-, all from PIE root *ne- "not."

In Old French and Middle English often en-, but most of these forms have not survived in Modern English, and the few that do (enemy, for instance) no longer are felt as negative. The rule of thumb in English has been to use in- with obviously Latin elements, un- with native or nativized ones.