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,
   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.
      [1913 Webster]

   2. United States currency; dollar bills; greenbacks. [slang]
      [PJC]

   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.
      [1913 Webster]

WordNet (r) 3.0 (2006):

lettuce
    n 1: informal terms for money [syn: boodle, bread,
         cabbage, clams, dinero, dough, gelt, kale,
         lettuce, lolly, lucre, loot, moolah, pelf,
         scratch, shekels, simoleons, sugar, wampum]
    2: any of various plants of the genus Lactuca
    3: leaves of any of various plants of Lactuca sativa

The Devil's Dictionary (1881-1906):

LETTUCE, n.  An herb of the genus _Lactuca_, "Wherewith," says that
pious gastronome, Hengist Pelly, "God has been pleased to reward the
good and punish the wicked.  For by his inner light the righteous man
has discerned a manner of compounding for it a dressing to the
appetency whereof a multitude of gustible condiments conspire, being
reconciled and ameliorated with profusion of oil, the entire
comestible making glad the heart of the godly and causing his face to
shine.  But the person of spiritual unworth is successfully tempted to
the Adversary to eat of lettuce with destitution of oil, mustard, egg,
salt and garlic, and with a rascal bath of vinegar polluted with
sugar.  Wherefore the person of spiritual unworth suffers an
intestinal pang of strange complexity and raises the song."