The Collaborative International Dictionary of English v.0.48:

Sleek \Sleek\ (sl[=e]k), a. [Compar. Sleeker (sl[=e]k"[~e]r);
   superl. Sleekest.] [OE. slik; akin to Icel. sl[imac]kr, and
   OE. sliken to glide, slide, G. schleichen, OHG. sl[imac]hhan,
   D. slik, slijk, mud, slime, and E. slink. Cf. Slick,
   Slink.]
   1. Having an even, smooth surface; smooth; hence, glossy; as,
      sleek hair. --Chaucer.
      [1913 Webster]

            So sleek her skin, so faultless was her make.
                                                  --Dryden.
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   2. Not rough or harsh.
      [1913 Webster]

            Those rugged names to our like mouths grow sleek.
                                                  --Milton.
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The Collaborative International Dictionary of English v.0.48:

Sleek \Sleek\, adv.
   With ease and dexterity. [Low]
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Sleek \Sleek\, n.
   1. That which makes smooth; varnish. [R.]
      [1913 Webster]

   2. A slick.
      [Webster 1913 Suppl.]

The Collaborative International Dictionary of English v.0.48:

Sleek \Sleek\, v. t. [imp. & p. p. Sleeked;p. pr. & vb. n.
   Sleeking.]
   To make even and smooth; to render smooth, soft, and glossy;
   to smooth over.
   [1913 Webster]

         Sleeking her soft alluring locks.        --Milton.
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         Gentle, my lord, sleek o'er your rugged looks. --Shak.
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WordNet (r) 3.0 (2006):

sleek
    adj 1: well-groomed and neatly tailored; especially too well-
           groomed; "sleek figures in expensive clothes"
    2: designed or arranged to offer the least resistant to fluid
       flow; "a streamlined convertible" [syn: streamlined,
       aerodynamic, flowing, sleek]
    3: having a smooth, gleaming surface reflecting light; "glossy
       auburn hair"; "satiny gardenia petals"; "sleek black fur";
       "silken eyelashes"; "silky skin"; "a silklike fabric"; "slick
       seals and otters" [syn: satiny, sleek, silken, silky,
       silklike, slick]
    v 1: make slick or smooth [syn: slick, sleek]

Moby Thesaurus II by Grady Ward, 1.0:

140 Moby Thesaurus words for "sleek":
   adipose, anal, balmy, blooming, blossoming, blubbery, booming,
   braw, buff, buffed, burnish, burnished, buttery, butyraceous, chic,
   chrismal, chrismatory, classy, clear, clothes-conscious,
   cosmopolitan, dapper, dashing, dinky, dressed to advantage,
   dressed to kill, elegant, exquisite, exuberant, fair, fat, fatty,
   fawning, finish, finished, flourishing, flowering, fruiting,
   furbish, furbished, genteel, glace, glance, glassy, glaze, glazed,
   gleaming, glistening, gloss, glossy, going strong, graceful,
   greasy, halcyon, hypocritical, in full swing, in good case, jaunty,
   lacquered, lardaceous, lardy, luster, lustrous, mucoid, natty,
   neat, nifty, nobby, oily, oleaginous, oleic, palmy, perfect,
   piping, polish, polished, posh, prospering, recherche, refine,
   rich, ritzy, rosy, round, rub, rubbed, satiny, scour, sebaceous,
   sharp, shellacked, shine, shining, shiny, shipshape, silken, silky,
   slick, slick down, slimy, slippery, smarmy, smart, smooth, smug,
   snazzy, snug, soapy, soigne, soignee, sophisticated, specious,
   spiffy, spruce, streamlined, style-conscious, suave, suety, swank,
   swanky, swell, tallowy, thriving, tidy, tight, tricksy, trig, trim,
   unctuous, unguent, unguentary, unguentous, varnish, varnished,
   velvety, vigorous, wax, well-cared-for, well-dressed,
   well-groomed