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1.
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?

Darren: GYFNO

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!
by The Noam Gnome August 25, 2011