Adj. Used to describe an object so horrific that it becomes seared into your retinas such that it can never be forgotten.
See also: goatse, tubgirl, What the fuck, goggles
Hey, look what I found on /b/.
>>OH GOD, YOU CAN'T UNSEE THIS SHIT, ASSHOLE.
Verb. To undo the act of seeing something; to erase the memory of having seen something, or otherwise reverse the effect of having seen something.
Internet meme: What has been seen cannot be unseen.