The opposite of underexaggeration (truth).
Overexaggeration differs from mesoexaggeration (commonly known simply as "exaggeration") in that mesoexaggeration strays from the truth to balance out skepticism, while overexaggeration goes beyond this to a non-believable, completely ridiculous extreme.
Tim: Me and my brother Joe took out four guys at this UFC fight thing. I can bench press 225 pounds. I got a 25...wait a 26...wait a 27 on the ACT.
Me: Tim, let's have a chat about this thing called exaggeration. Or, in your case, overexaggeration.