Definition of thin verb from the Oxford Advanced American Dictionary

     

    thin

     verb
    verb
    NAmE//θɪn//
     
    Verb Forms present simple I / you / we / they thin
     
    he / she / it thins
     
    past simple thinned
     
    -ing form thinning
     
     
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    liquid
  1. 1[transitive] thin something (down) (with something) to make a liquid less thick or strong by adding water or another substance Thin the paint with water.
  2. of hair
  3. 2[intransitive] to become less thick a middle-aged man with thinning hair
  4. become less thick
  5. 3 [intransitive, transitive] to become less thick or fewer in number; to make something less thick or fewer, for example by removing some things or people The clouds thinned and the moon shone through. Global warming could be accelerated due to the thinning of the ozone layer. thin out The crowd had thinned out and only a few people were left. thin something (out) Thin out the seedlings to about 4 inches apart.
See the Oxford Advanced Learner's Dictionary entry: thin