Lac Leman is the name by which Lake Geneva is known in Lausanne, Switzerland.
There Eliot finished the draft of
The Waste Land
while undergoing treatment for a case of nerves.
Leman is an archaic English word for lover or paramour.
Also, this line could be an allusion to Psalm 137,
Shakespeare's The Tempest,
or to the Wittelsbach family history.
See more at the