The concept where overweight and obese Tumblr users (usually white women, but not limited to race, gender, or sex) whine about how being fat is equal to the years of oppression similar to racism. More accurately, an ongoing game of sour grapes, wherein those too lazy to change themselves will instead blame society for the fact that they're missing out on all the best parts of life.
Many proponents of "thin privilege" also make claims that there are no adverse effects of being extremely overweight or obese.
400-lb woman: I can't fit into an airplane seat so I can't go to the All-You-Can-Eat Convention unless I buy two seats!
Friend: omg THIN PRIVILEGE!!!!
400-lber: Fat is a feminist issue! I can't help that I'm fat they're oppressing meeeee!! Oh hey pass the chips.
(this definition pertains specifically to Tumblr user "weetz")
A fat, angry fat acceptance blogger who will attempt to prove to the internet how happy, fulfilled, and perfect she is by telling everyone over and over and over again that that's the case. This makes it extremely obvious she's only trying to convince herself she's happy. Everyone else catches on immediately that she's miserable.
Will tell you a million times over that she's NOT WHITE OK? and also is involved in the very controversial Orange Order. Types in all caps, for "stylistic" reasons.
Complete joke and worth poking with a stick when you're bored.
Weetz posts on Tumblr looking for validation 24/7. Only other miserably insecure fatties care.