Synonyms for Thin:

adj.

deficient, weak (adjective)
feeble, flat, implausible, improbable, inadequate, inconceivable, incredible, insubstantial, insufficient, poor, scant, scanty, scarce, shallow, sketchy, sparse, thick, transparent, unbelievable, vapid.
dilute (adjective)
Waterish.
diluted (adjective)
dilute, fine, light, watery, weak, watered-down.
exiguous (adjective)
meager, tiny.
feeble (adjective)
insufficient.
few (adjective)
few.
fine, light, slender (adjective)
attenuate, attenuated, bony, cadaverous, emaciated, ethereal, gangling, gangly, gaunt, haggard, lank, lanky, lean, meager, narrow, pinched, puny, rangy, rarefied, rawboned, reedy, rickety, scraggy, scrawny, shriveled, skeletal, skinny, slight, slim, spare, spindly, threadlike, twiggy, twiglike, underweight, wan, wasted, wizened.
gangling (adjective)
gangling, gangly, Spindling.
insufficient (adjective)
inadequate.
moist (adjective)
watery.
narrow (adjective)
fine, lanky, lean, narrow, scrawny, skeletal, skinny, slender, slim, tenuous, tight, willowy.
rangy (adjective)
reedy.
transparent, fine (adjective)
diaphanous, filmy, flimsy, gossamer, permeable, rare, sheer, tenuous, unsubstantial, wafer-thin, wispy.
wasted (adjective)
skeletal.
weak (adjective)
washy.

adv.

all (adverb)
thinly.

n.

across, attenuated, broad, elongated, heavy, permeable, thick. adenoidal, appealing, brittle, croaky, dead, feeble, fruity, grating, insubstantial, shallow, sketchy, solid, vapid, breathy. aerate, bodiless, bonsai, compost, cross-fertilize, cross-pollinate, deadhead, dense, disembodied, drill, ethereal, faint, feed, piping, shaky, shrill, tiny, bed out. air, airborne, airflow, airless, airy, angular, atmosphere, blast, current, fat, meager, Breathable, Fleshless. strong, washy, watery, Waterish, watered-down, be (all) skin and bone(s). barely, big, excess, exiguous, few, measly, mere, minimum, only, poor, puny, scant, scanty, stingy. aquatic, aqueous, bubbly, crystalline, fizzy, fluid, gaseous, gassy, implausible, improbable, inconceivable, incredible, likely, liquid, powdered, unbelievable, unsubstantial, Unconceivable. atomize, bake, boil, boil over, break down, bubble, cloud, boil away, boil up. baby-faced, chiseled, craggy, full-face, furrowed, good-looking, fresh-faced, hatchet-faced. bald, balding, body, bouffant, bristly, bushy, close-cropped, bad hair day, bedhead, coiffed. beneath, clothed, crusted, encrusted, thickly, under, underneath, Caked, be tipped with something. blow-by-blow, descriptive, detailed, edited, elaborate, extensive, general, 101, broad-brush. broadly, cheesy, gummy, humorless, humourless, lazy, mirthless, MONA LISA. compact, condense, constrict, dwarf, let down, shrivel, scale back. contraction, retract, shrinkage, wax and wane. flow, gray, hang, recede, rise, tumble-down, wave, grow out. slender (noun)
fat, obese, rangy, rickety, wan, Spindling.
sparse (noun)
deficient, inadequate, insufficient, scarce.
thin (noun)
anorectic, anorexic, bladed, bony, cadaverous, capillary, chiffon, cobwebby, compressed, cut, deep-eyed, depressed, diaphanous, dilute, distributed, emaciated, filamentlike, filamentous, filiform, filmy, fine, flat, flimsy, gangling, gangly, gaunt, gauzy, gossamer, haggard, hairlike, hollow-eyed, hyperfine, lank, lanky, lean, light, lose weight, melt off, pale, papery, pinched, rare, rarefied, rarified, rawboned, reduce, reedlike, reedy, ribbonlike, ribbony, scarecrowish, scraggy, scrawny, sheer, shriveled, shrivelled, shrunken, skeletal, skinny, sleazy, slender, slender-waisted, slenderize, slight, slim, slim down, slim-waisted, spare, sparse, spindle-legged, spindle-shanked, spindly, spiritless, stringy, sunken-eyed, tenuous, thin out, thinly, threadlike, thready, transparent, trim, twiggy, twiglike, underweight, vaporous, wafer-thin, wasp-waisted, wasted, weak, weedy, wiry, wisplike, wispy, withered, wizen, wizened, see through.

v.

dilute (verb)
attenuate, cut, dilute, reduce.
make diluted or less dense (verb)
cut, reduce, trim.
narrow (verb)
slenderize, tighten.

Other synonyms:

fat, few, measly, tumble-down, washy, Fleshless, Unconceivable, Waterish, watered-down, grow out, scale back. constrict, exiguous, faint, implausible, incredible, liquid, retract, shrinkage. angular, condense, contraction, improbable, inconceivable, puny, recede, scant, stingy, unbelievable. shrivel. light. dwarf, shaky. compact, let down. poor. gray. hang. wave. rise. delicate
airy.
flimsy
flimsy, unsubstantial.
high-pitched
shrill.
scarce
scanty.
skinny
rangy, Spindling.
thin
meager.
weaken
attenuate.
Other relevant words:
attenuated, exiguous, flimsy, gaunt, insubstantial, meager, poor, scant, scanty, sheer, shrill, slight, spare, sparse, thin out, thinly, washy, watery, weak.

Usage examples for thin

  1. It was that long, thin boy yonder. – Burr Junior by G. Manville Fenn
  2. My child, how thin you've grown! – The Twilight of the Souls by Louis Couperus
  3. And how thin you are! – The Martian by George Du Maurier