Say or write and send away rapidly, as in He fired off three more questions, or She fired off a letter of complaint to the president. This expression originally (from about 1700) was, and still is, used in the sense of “discharge a weapon or ammunition,” as in The police were instructed to fire off canisters of tear gas. The figurative use dates from the late 1800s.
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.