The Collaborative International Dictionary of English v.0.48:

Slender \Slen"der\, a. [Compar. Slenderer; superl.
   Slenderest.] [OE. slendre, sclendre, fr. OD. slinder thin,
   slender, perhaps through a French form; cf. OD. slinderen,
   slidderen, to creep; perh. akin to E. slide.]
   1. Small or narrow in proportion to the length or the height;
      not thick; slim; as, a slender stem or stalk of a plant.
      "A slender, choleric man." --Chaucer.
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            She, as a veil down to the slender waist,
            Her unadorned golden tresses wore.    --Milton.
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   2. Weak; feeble; not strong; slight; as, slender hope; a
      slender constitution.
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            Mighty hearts are held in slender chains. --Pope.
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            They have inferred much from slender premises. --J.
                                                  H. Newman.
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            The slender utterance of the consonants. --J. Byrne.
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   3. Moderate; trivial; inconsiderable; slight; as, a man of
      slender intelligence.
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            A slender degree of patience will enable him to
            enjoy both the humor and the pathos.  --Sir W.
                                                  Scott.
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   4. Small; inadequate; meager; pitiful; as, slender means of
      support; a slender pittance.
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            Frequent begging makes slender alms.  --Fuller.
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   5. Spare; abstemious; frugal; as, a slender diet.
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            The good Ostorius often deigned
            To grace my slender table with his presence.
                                                  --Philips.
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   6. (Phon.) Uttered with a thin tone; -- the opposite of
      broad; as, the slender vowels long e and i.
      [1913 Webster] -- Slen"der*ly, adv. -- Slen"der*ness,
      n.
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WordNet (r) 3.0 (2006):

slender
    adj 1: being of delicate or slender build; "she was slender as a
           willow shoot is slender"- Frank Norris; "a slim girl with
           straight blonde hair"; "watched her slight figure cross
           the street" [syn: slender, slight, slim, svelte]
    2: very narrow; "a thin line across the page" [syn: slender,
       thin]
    3: having little width in proportion to the length or height; "a
       slender pole"
    4: small in quantity; "slender wages"; "a slim chance of
       winning"; "a small surplus" [syn: slender, slim]
    5: moving and bending with ease [syn: lissome, lissom,
       lithe, lithesome, slender, supple, svelte,
       sylphlike]

Moby Thesaurus II by Grady Ward, 1.0:

191 Moby Thesaurus words for "slender":
   Junoesque, Lenten, Spartan, abstemious, agreeable, airy,
   amply endowed, angustifoliate, angustirostrate, angustisellate,
   angustiseptal, ascetic, asinine, attenuate, attenuated, austere,
   becoming, bonny, boyish, braw, built, built for comfort, buxom,
   callipygian, callipygous, catchpenny, circumscribed, close,
   close-fitting, comely, confined, constricted, cramp, cramped,
   crowded, curvaceous, curvy, deficient, delicate, diaphanous,
   diluted, dwarfed, dwarfish, empty, ethereal, exiguous, fair, fat,
   fatuous, feeble, fine, fine-drawn, finespun, flimsy, foolish,
   frail, fribble, fribbling, frivolous, frothy, frugal, futile,
   gauzy, girlish, goddess-like, good-looking, goodly, gossamer,
   graceful, gracile, idle, impoverished, inadequate, inane,
   incapacious, incommodious, insignificant, insubstantial,
   insufficient, isthmian, isthmic, jejune, lacy, lanky, lean, light,
   likely, limited, lissome, lithe, little, lovely to behold, meager,
   mean, miserly, misty, narrow, near, negligible, niggardly,
   nugacious, nugatory, off, otiose, outside, paltry, papery,
   parsimonious, personable, pleasing, pneumatic, poor, presentable,
   puny, rare, rarefied, reedy, remote, restricted, scant, scanty,
   scarce, scrawny, scrimp, scrimpy, shallow, shapely, shy, sightly,
   silly, skimp, skimpy, slenderish, slight, slight-made, slim,
   slimmish, slinky, small, spare, sparing, stacked, starvation,
   statuesque, stingy, stinted, strait, straitened, stunted,
   subsistence, subtle, superficial, svelte, sylphlike, tenuous, thin,
   thin-bodied, thin-set, thin-spun, thinnish, threadlike, tight,
   trifling, trim, trite, trivial, twiggy, unlikely, unnourishing,
   unnutritious, vacuous, vague, vain, vapid, wanting, wasp-waisted,
   watered, watered-down, watery, weak, well-built, well-favored,
   well-formed, well-made, well-proportioned, well-shaped,
   well-stacked, willowy, windy, wiredrawn, wispy