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.)