thin
I
 /ˈθın/  adj  , thin·ner, -nest [also more ~; most ~]
  1 : having a small distance between the top and bottom or front and back surfaces : not thick
   thin paper/walls
   a thin band/line/stripe
   a thin coating/layer of dust
   pizza with a thin crust
   a thin slice of ham
  2 : not having a lot of extra flesh on the body : not fat
   a thin child
   She is very/too thin.
   thin legs
  synonyms see lean, 2
  3 a : not growing closely together
   a thin stand of trees
  : not growing in a large amount
   thin hair
   His hair is a little thin on top. = He's a little thin on top. [=he does not have a lot of hair on the top of his head]
  3 b : having less than the usual, original, or needed number or amount
   Attendance at the carnival was a bit thin. [=low]
   My patience was wearing/running thin. [=I was becoming less patient]
  4 of a liquid : flowing very easily : containing a lot of water
   thin broth/gravy/soup
  5 : having less oxygen than normal
   thin air
   a thin atmosphere
  6 : easy to see through
   The fog was relatively thin.
   a thin mist
  7 : not large or impressive
   a thin [=very small] margin of victory
   She has a thin [=narrow] lead in the polls.
  8 : not very good, useful, etc. : weak
   thin arguments/explanations
   That excuse is pretty thin.
   The story's plot is pretty thin.
   His argument quickly wore thin.
   a thin smile [=a weak smile that does not seem sincere]
  9 of a voice : weak and high
   a high, thin voice
  (a) thin skin — see skin, 1
  disappear/vanish into thin air
   : to disappear completely in a way that is mysterious
    The papers seem to have vanished into thin air.
  have a thin time (of it)
    Brit   : to be in a difficult situation especially because you do not have enough money
    He was having a thin time of it until his novel started selling.
  on thin ice — see ice, 1
  out of thin air
    — used to say that someone or something appears in a sudden and unexpected way
    He appeared out of thin air. [=out of nowhere]
   
  the thin end of the wedge
    Brit   : the beginning of something that will become more serious, unpleasant, etc.
    The bank's decision to raise rates could be the thin end of the wedge. [=many other banks may also decide to raise rates]
  thin on the ground
    Brit   : existing or occurring in very small amounts
    Useful suggestions have been thin on the ground lately. [=there have been very few useful suggestions lately]
  through thick and thin — see thick, 3
  thin·ness /ˈθınnəs/  noun   [  noncount  ]
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II
  verb  , thins, thinned, thin·ning
  : to make (something or someone) thin or thinner or to become thin or thinner: such as
   a : to become less crowded, close together, or full
   [  no obj  ] The crowd gradually thinned [=grew smaller] as the night went on.
    The trees thinned as we climbed higher. [=there were fewer trees as we climbed higher]
   [  + obj  ] The bad weather thinned (out) the crowd a little.
    He plans to thin (out) the young carrots in the garden. [=to remove some of them so that there is more room between the remaining carrots]
   b [  + obj  ] : to make (a liquid) less thick by adding water or some other liquid to it
   He added a little more water to thin the gravy.
   c [  no obj  ] of a person's hair : to become less thick as more hairs fall out over time
   His hair is thinning. = He has thinning hair.
   d [  no obj  ] of fog, smoke, etc. : to become easier to see through : to become less thick or dense
   The haze thinned in the late afternoon.
   e : to become less fat or to make (someone) less fat
   [  no obj  ]
    — usually + down
    She's thinned down a lot in the past year.
   
   [  + obj  ] His face has been thinned by illness.
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III
  adv  , thin·ner, -nest [also more ~; most ~]
  : in a way that makes thin pieces, layers, etc.
   She sliced the cheese thin.