When someone looks good in a thumbnail picture, and you click on it only to find out they don't look nearly as good in the full picture, you've just been thumbnailed.
"Ooh, she looks hot."
"Dammit! Thumbnailed again!"
That feeling in your stomach that you get after looking at a hot girl's thumbnail on Facebook, clicking on it, and realizing that she's ugly as shit.
Person 1: Bro, she looks so hot. Click on her picture.
Person 2: LOL ALRIGHT
Person 1: DAMN, This computer is super slow.
Person 2: SHUT UP DOOD LOL
Person 1: AHHH WTF SHE'S UGLY AS SHIT
Person 2: HAHAHA HOW DOES YOUR ASS FEEL? YOU JUST GOT THUMBNAILED.