early 15c., probacioun, "trial, experiment, test, act of examining with a probe," 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 of a candidate for membership) is from early 15c., originally especially a testing or trial by adversity. The theological sense is recorded by 1520s; the criminal justice sense is recorded by 1866. As a verb from 1640s. Related: Probationer; probationary.