IPA: /ˈærənt/
KK: /ˈærənt/
Completely or thoroughly something, often used to emphasize a negative quality.
He is an arrant fool for believing that lie.
Arrant comes from the Old French 'errant', meaning wandering or straying, which is derived from the Latin 'errans', the present participle of 'errare', meaning to wander or stray. The word originally described someone who was wandering or straying, often used in a negative sense to imply someone who is notorious or without merit.
Think of someone who is 'wandering' or 'straying' — that's why 'arrant' describes someone who is notorious or without merit.