In imminent danger, in an explosive situation, as in Our office is sitting on a powder keg while management decides whether or not to close us down. This metaphoric term alludes to sitting on a keg of gunpowder that could go off at any moment. [First half of 1900s]
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.