keep one's powder dry
Stay alert, be careful, as in Go ahead and take on the opposition, but keep your powder dry. This colloquial expression, which originally alluded to keeping gunpowder dry so that it would ignite, has been used figuratively since the 1800s but today is less common than take care.
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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.