(1) Wandering or moving about aimlessly. (2) Straying outside proper bounds, or away from an accepted pattern or standard.
Example:
Modern-day cowboys have been known to use helicopters to stop errant calves.
Errant means both "wandering" and "mistaken." A knight-errant was a wandering knight going about slaying dragons or rescuing damsels in distress. Arrant is a rarely used variant of errant, but we sometimes hear it in the phrase arrant knave, which comes from Shakespeare and refers to an extremely untrustworthy individual. More typical is the errant cloud or breeze that just happens along or the errant child that requires discipline.