What is "lettuce opium"

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, Lactucic.]

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 Lamb.

   Lettuce opium. See Lactucarium.

   Sea lettuce, certain papery green seaweeds of the genus Ulva.

alt. lactucarium. n. lactucarium.