beg to differ - 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.
Disagree with someone, as in John told me Max was sure to win, but I beg to differ—I don't think he has a chance. This courteous formula for expressing disagreement echoes similar uses of beg in the sense of “ask,” such as I beg your pardon, so used since about 1600. Also see excuse me.