Idioms and Phrases with rest assured
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® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.