Synonyms for Thin:

adj.

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

adv.

all (adverb)
thinly.

n.

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

v.

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

Other synonyms:

tumble-down, measly, grow out, Fleshless, Unconceivable, scale back, fat, Waterish, few, washy, watered-down. constrict, incredible, implausible, faint, retract, exiguous, liquid, shrinkage. improbable, inconceivable, unbelievable, contraction, scant, puny, stingy, angular, condense, recede. 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:
poor, scant, meager, gaunt, sparse, thinly, washy, watery, exiguous, scanty, thin out, insubstantial, attenuated, slight, sheer, weak, spare, flimsy, shrill.

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