Discharge a missile or fire a weapon; also, attack verbally. For example, He let fly a rotten egg at the speaker, or They let fly some insults laced with four-letter words. The first usage dates from about a.d. 1000, the second from the late 1500s.
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.