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thin something out
to make something less dense; to scatter something. You will have to thin the young plants out, because there is not room for all of them. Can you thin out these young plants?
to spread out; to become less dense. The trees began to thin out as we got higher up the mountain. The crowd began to thin out as we got a little farther from the theater.
1. To make something less dense or concentrated: A harsh winter thinned out the herd of deer. The chef thinned the sauce out with water.
2. To become less dense or concentrated: The air thinned out as we climbed up the mountain.