to say the least - Phrases/Idioms
The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 2003, 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Not to exaggerate, as in When the ring turned up in the lost and found, she was delighted, to say the least. This idiom was first recorded in 1809.