on a dime
In a very small space, suddenly, as in That horse is so well trained it can turn on a dime. This expression alludes to the fact that the dime is the smallest-size U.S. coin. [Early 1900s]
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.