The Collaborative International Dictionary of English v.0.48:

Treacle \Trea"cle\ (tr[=e]"k'l), n. [OE. triacle a sovereign
   remedy, theriac, OF. triacle, F. th['e]riaque (cf. Pr.
   triacla, tiriaca, Sp. & It. triaca, teriaca), L. theriaca an
   antidote against the bite of poisonous animals, Gr. ?, fr. ?
   of wild or venomous beasts, fr. qhri`on a beast, a wild
   beast, dim. of qh`r a beast. Cf. Theriac.]
   1. (Old Med.) A remedy against poison. See Theriac, 1.
      [1913 Webster]

            We kill the viper, and make treacle of him. --Jer.
                                                  Taylor.
      [1913 Webster]

   2. A sovereign remedy; a cure. [Obs.]
      [1913 Webster]

            Christ which is to every harm treacle. --Chaucer.
      [1913 Webster]

   3. Molasses; sometimes, specifically, the molasses which
      drains from the sugar-refining molds, and which is also
      called sugarhouse molasses.
      [1913 Webster]

   Note: In the United States molasses is the common name; in
         England, treacle.
         [1913 Webster]

   4. A saccharine fluid, consisting of the inspissated juices
      or decoctions of certain vegetables, as the sap of the
      birch, sycamore, and the like.
      [1913 Webster]

   Treacle mustard (Bot.), a name given to several species of
      the cruciferous genus Erysimum, especially the Erysimum
      cheiranthoides, which was formerly used as an ingredient
      in Venice treacle, or theriac.

   Treacle water, a compound cordial prepared in different
      ways from a variety of ingredients, as hartshorn, roots of
      various plants, flowers, juices of plants, wines, etc.,
      distilled or digested with Venice treacle. It was formerly
      regarded as a medicine of great virtue. --Nares.

   Venice treacle. (Old Med.) Same as Theriac, 1.
      [1913 Webster]

WordNet (r) 3.0 (2006):

treacle
    n 1: a pale cane syrup [syn: treacle, golden syrup]
    2: writing or music that is excessively sweet and sentimental
       [syn: treacle, mush, slop, glop]

Moby Thesaurus II by Grady Ward, 1.0:

71 Moby Thesaurus words for "treacle":
   albumen, ambrosia, artificial sweetener, batter, blackstrap,
   bonnyclabber, butter, calcium cyclamate, cane syrup, clabber,
   clover honey, comb honey, corn syrup, cornstarch, cream, curd,
   cyclamates, dough, edulcoration, egg white, gaum, gel, gelatin,
   glair, glop, glue, gluten, goo, gook, goop, gruel, gumbo, gunk,
   honey, honeycomb, honeydew, honeypot, jam, jell, jelly, loblolly,
   maple syrup, molasses, mucilage, mucus, nectar, pap, paste,
   porridge, pudding, pulp, puree, putty, rob, saccharification,
   saccharin, semifluid, semiliquid, size, sodium cyclamate, sorghum,
   soup, starch, sticky mess, sugar, sugar-making, sugaring off,
   sweetener, sweetening, sweets, syrup