stand guard
Watch over, act as a lookout, as in We'll climb the tree and get the apples if Josh will stand guard, or There's a parking space; stand guard while I make a U-turn and get to it. This term alludes to the military defense of posting guards to watch for the enemy.
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The American Heritage® Idioms Dictionary
Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.