When one of your front teeth is freakishly alligned with your septum.
Check out Tom Cruise and his fugly unitooth!
A freakish row of teeth that, like a unibrow, seems to consist of a single mass of unbroken, tooth-like substance when a person smiles. Akin to a mouthguard made with a single, gigantic tooth.
I am sick of seeing Tiger Woods and his ridiculous unitooth on TV all the time.