The Collaborative International Dictionary of English v.0.48:

Viscous \Vis"cous\, a. [L. viscosus. See Viscid.]
   Adhesive or sticky, and having a ropy or glutinous
   consistency; viscid; glutinous; clammy; tenacious; as, a
   viscous juice. -- Vis"cous*ness, n.
   [1913 Webster]

   Note: There is no well-defined distinction in meaning between
         viscous and viscid.
         [1913 Webster]

WordNet (r) 3.0 (2006):

viscous
    adj 1: having a relatively high resistance to flow [syn:
           syrupy, viscous]
    2: having the sticky properties of an adhesive [syn: gluey,
       glutinous, gummy, mucilaginous, pasty, sticky,
       viscid, viscous]

Moby Thesaurus II by Grady Ward, 1.0:

94 Moby Thesaurus words for "viscous":
   adhesive, amylaceous, broad-bodied, bulky, bullnecked, clabbered,
   clammy, close, close-knit, close-textured, close-woven, clotted,
   coagulated, coarse, compact, compacted, compressed, concentrated,
   concrete, condensed, congested, consolidated, corpulent, crammed,
   crammed full, crass, crowded, curdled, dense, doughy, fat, firm,
   full, full-bodied, gaumy, gelatinous, glairy, gluelike, gluey,
   glutenous, glutinose, glutinous, gooey, gross, grumous, gumbo,
   gumbolike, gumlike, gummous, gummy, hard, heavy, heavyset,
   impenetrable, impermeable, inspissated, jam-packed, jammed, jelled,
   jellied, jellylike, massive, mucilaginous, nonporous, packed,
   pasty, ropy, semifluid, serried, slab, slabby, slimy, slithery,
   solid, starchy, sticky, stiff, stodgy, stringy, substantial,
   syrupy, tacky, tenacious, thick, thick-bodied, thick-growing,
   thickened, thickset, three-dimensional, tough, tremelloid,
   tremellose, viscid, viscose