A female phantom, whose name was used by the Greeks and Romans as a bugbear to children. She was a Libyan queen beloved by Jupiter, but robbed of her offspring by the jealous Juno; and in consequence she vowed vengeance against all children, whom she delighted to entice and murder. (See Fairy.)
“Keats has a poem so called. His Lamia is a serpent who assumed the form of a beautiful woman, was beloved by a young man and got a soul. The tale was drawn from Philostratus.” —De Vita Apollonii book iv., introduced by Burton in his Anatomy of Melancholy.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894