The opposite of "over exaggerate".
To claim something is a lot less/smaller than it really is.
Can also be used in the forms "Underaggerating" and "Underaggerate"
Kathryn: How much do you get paid?
Kathryn: Come on, tell me. You know I'll find out anyway.
Darren: If you must know, $70,000 per year
Kathryn: That's the worst case of underaggeration I've ever heard!