In rapid retreat; also, attempting to escape from pursuers. For example, The guerrillas were on the run after the ambush, or The burglars were on the run from the police. [Early 1800s]
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.