E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Wan means thin.
(Anglo-Saxon, wan, deficient; our wane, as the waning moon.) As wasting of the flesh is generally accompanied with a grey pallor, the idea of leanness has yielded to that of the sickly hue which attends it. (Verb wan-ian, to wane.)