The Collaborative International Dictionary of English v.0.48:
Lettuce \Let"tuce\ (l[e^]t"t[i^]s), n. [OE. letuce, prob.
through Old French from some Late Latin derivative of L.
lactuca lettuce, which, according to Varro, is fr. lac,
lactis, milk, on account of the milky white juice which flows
from it when it is cut: cf. F. laitue. Cf. Lacteal,
1. (Bot.) A composite plant of the genus Lactuca (Lactuca
sativa), the leaves of which are used as salad. Plants of
this genus yield a milky juice, from which lactucarium is
obtained. The commonest wild lettuce of the United States
is Lactuca Canadensis.
2. United States currency; dollar bills; greenbacks. [slang]
Hare's lettuce, Lamb's lettuce. See under Hare, and
Lettuce opium. See Lactucarium.
Sea lettuce, certain papery green seaweeds of the genus