In modern English “arrant” is usually used to describe someone notorious, thoroughly shameless: an arrant villain, an arrant thief. It has a rather old-fashioned air to it, and is often used in antique phrases like “an arrant knave.”

“Errant” is also an antique word, now used exclusively to mean “wandering,” especially in the phrase “a knight errant” (a wandering knight). As here, it usually follows the noun it modifies. Although you can argue that “arrant” also used to have this meaning, most readers will regard its use in this sense today not as learned, but as mistaken.