More than just the opposite of exaggeration , an inaggeration is the removal of the colloquial interest which can be added to an otherwise boring sentence. To exaggerate would be to say you were paid a million dollars for a job you got 10,000 for, an inaggeration would be to say anywhere between 9000 and 11000. It is not exactly a statement of truth but it isn't incorrect enough to add anything to the story. More extreme cases of inaggeration are known as Hypoboles.
Guy 1: 'Chuck Norris is so strong that he can lift an automobile.'
Guy 2: 'Nobody has said a Chuck Norris joke in 5 years and you expect to revive them with that hypobole piece of shit. If you inaggerate once more i'm going to stick my foot so far up your ass that it will hurt a little'
The opposite of Exaggerate.
"How much money did you get for that job?"
"Not that much..."
"Oh,come one, don't inaggerate, I thought it was a big job."
"I only miss you a little"
"You are good at inaggerating."