thin3 past tense and past participle thinned, present participle thinning
1 [intransitive and transitive] also thin out
to become fewer in number, especially when there were many before, or to remove people, plants, or things so that fewer remain:
The crowd had thinned out and only a few people were left.
The trees thinned as we got closer to the top of the mountain.
Traffic was finally thinning.
Thin the carrots to two inches apart.
Her hair had been thinned and cut shorter.
2 [intransitive and transitive]
to make something thinner or to become thinner [≠ thicken]:
The clouds had begun to thin.
A narrow smile thinned his lips.
3 [transitive] also thin down
to make a liquid weaker by adding water or another liquid:
Thin the sauce by adding milk.
thin something with something
The pastels can be thinned with water.
if someone's hair is thinning, they have less hair than they used to:
a tall man with thinning hair
if something thins the ranks of a group of people, there are fewer of them as a result of it:
Illness had thinned our ranks.