Late 16th century; earliest use found in Thomas Twyne (1543–1613), physician. From Welsh afanc, †afangc (also †addanc) aquatic monster, beaver from the same Celtic base (with a velar suffix) as Welsh afon, Middle Breton, Breton aven, Early Irish ab (n-stem), all in sense ‘river’ from the same Indo-European base as classical Latin amnis river.