Idioms and Phrases with put it mildly
put it mildly
Understate, say without exaggeration, as in It's a fairly long way to walk, to put it mildly—twenty miles or so. [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.