Calm down, get hold of oneself. For example, Before you tell him what you think of him, count to ten. Often used as an imperative, this phrase in effect means that if one takes the time to count from one to ten one can regain one's composure.
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.