rest assured

rest assured idiom
You can be sure, as in Rest assured that the police will recover your diamonds. This expression uses assured in the sense of “certain” or “confident,” a usage dating from the early 1500s.

The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 2003, 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.

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