(1) thin down指“稀释，(使)数目减少，(使)瘦下来”。

• You'd better thin the paint down so that it spreads more easily.
  你最好把油漆稀释一下，这样才好刷开。
• Mary has thinned down a lot since last year.
  从去年以来玛丽瘦下来不少。

 (2) thin out意为“间(苗)，使变稀疏，瘦下来”。

• The old man is in the garden thinning out the onion plants; he should have a good crop this year.
  老人正在园子里给洋葱间苗；他今年会有个好收成。
• The crowd thinned out/down when all the excitement was over.
  骚动过后，人群逐渐散去了。