arrant

A variant of errant, from Middle English erraunt [and other forms], from Anglo-Norman erraunt, from Old French errant, the present participle of errer (“to walk (to); to wander (to); (figuratively) to travel, voyage”), and then: * from Vulgar Latin iterō (compare Late Latin itinerō, itineror (“to travel, voyage”)), from Latin iter (“a route (including a journey, trip; a course; a path; a road)”), ultimately from Proto-Indo-European *h₁ey- (“to go”); and * from Latin errāns (“straying, errant; wandering”), the present active participle of errō (“to rove, wander; to get lost, go astray; to err, wander from the truth”), ultimately from Proto-Indo-European *h₁ers- (“to flow”). The original sense was sense 3 (“roving around, wandering”). Due to the word being used to describe disreputable persons who wandered about (for example, arrant knave and arrant thief), it came to be used as an intensifier (sense 1: “complete; downright; utter”) and to have a negative meaning (sense 2: “very bad; despicable”).

来源：wiktionary