Guy1: 2+2=4
Guy2: I beg to differ

ex. (talking to a friend on fbook chat)

person 1: hey man whats going on tonight
person 2: i gotta go to the bathroom il poopt you

or

person 1: whatsup man what are you doing?
person 2: poopting you

I beg to differ about Mr Johnson's final assertion.

Example:

Joe: Hey I don't think that's true
Jane: I beg to differ
Joe: No like it's literally on their website saying it isn't true.