Probable
late Middle English (in the sense ‘worthy of belief’): via Old French from Latin probabilis, from probare ‘to test, demonstrate’.
wiktionary
From Middle English probable, from Old French probable, from Latin probābilis(“that may be proved, credible”), from probāre(“to test, examine”); see probe, prove. Compare recent doublet provable.
etymonline
probable (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.
