Late 16th century; earliest use found in William Fulke (d. 1589), theologian and college head. From classical Latin Achillēus of or resembling Achilles (from ancient Greek Ἀχίλλειος of Achilles from Ἀχιλλεύς (classical Latin Achillēs) Achilles, the name of a Greek hero who was invulnerable except in the heel + -ειος, suffix forming adjectives) + -an.