WordWeb Online

1. Make sparse
   "thin out the young plants"
    
2. Become sparser
   "Towards the end of town, the houses thinned out"
    
3. Lessen the strength or flavour of a solution or mixture
   - dilute, thin, reduce, cut

Derived forms: thinning out, thins out, thinned out

Type of: bring down, cut, cut back, cut down, decrease, diminish, fall, lessen, reduce, trim, trim back, trim down, weaken