Is it farther or further?
early 15c., "trial, experiment, test," from Old French probacion "proof, evidence" (14c., Modern French probation) and directly from Latin probationem (nominative probatio) "approval, assent; a proving, trial, inspection, examination," noun of action from past participle stem of probare "to test" (see prove). Meaning "testing of a person's conduct" (especially as a trial period for membership) is from early 15c.; theological sense first recorded 1520s; criminal justice sense is recorded by 1866. As a verb from 1640s. Related: Probationer; probationary.