early 13c., "protector, defender," from Old North French warant (Old French garant), from Frankish *warand (cf. Old High German weren "to authorize, warrant," German gewähren "to grant"), from Proto-Germanic *war- "to warn, guard, protect," perhaps from PIE root *wer- "to cover" (cf. Latin vereri "to observe with awe, revere, respect, fear;" Greek ouros "watchman," horan "to see;" Hittite werite- "to see;" see weir).
Sense evolved via notion of "permission from a superior which protects one from blame or responsibility" (c.1300) to "document conveying authority" (1510s). A warrant officer ...