The story
Uncertain, and subject to various attempts to connect it to multiple Germanic and Romance etymons; first attested in the First Folio, and sometimes posited to be an error for anoint, referring to a witch’s supposed flight-facilitating ointment, and thus being a commandment for her to anele herself and fly away; also said to be taken from the phrase rowan tree, thought to dispel witches.
Etymology from Wiktionary, CC BY-SA 4.0.