Aphetic formed on avouter. In its oldest form from Old French avoutre (Provencal avoutre, -ro) aöutre:—Latin adulterum (etymol. uncertain, perhaps from ad. to + alter other, different); afterwards assimilated to the Latin as advouter, ad-oulter (MFr. advoultre); so as to become at length in form a direct adoption of the Latin.
From: A New English Dictionary on Historical Principles, James Murray, 1888-1933