Webster's Revised Unabridged Dictionary
AS. ampre, ompre, a crooked swelling vein: cf. Prov. E. amper, a tumor with inflammation. Cf. the first syllable in agnail, and berry, a fruit
Chambers's Twentieth Century Dictionary
Often explained as a disguised form of A.S. ampre, a crooked swelling vein; more probably, a variant of anbury = angberry, A.S. ang-, pain, as in ang-nail.