Vivien, mistress of Merlin, the enchanter, who lived in the
midst of an imaginary lake, surrounded by knights and damsels.
Tennyson, in the Idylls of the King, tells the story of Vivien
and Merlin. (See Lancelot.)
Lady of the Lake.
Ellen Douglas, who lived with her father near Loch Katrine. (SirWalterScott: The Lady of the Lake.)
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894