9/14 -- Revolve issued an apology, saying, "We messed up big. We are SO SORRY for hurting and offending you." They go on to say they have canceled the line and are donating $20k to Girls Write Now -- the charity that was going to benefit from sales of the sweatshirt.

Lena Dunham is backing up the bus over Revolve ... saying the company essentially stabbed her in the back when it put her "Fat is not Beautiful" sweatshirt on a skinny white chick.

Lena says she had teamed with Revolve to roll out the shirts which read, "Fat is not beautiful it's an excuse." She says the intent was to reclaim the hateful phrases Internet trolls use -- a la rappers using the n-word.

But, she says that all backfired when the clothing company "presented the sweatshirts on thin white women." Lena says that destroyed any attempt to reclaim the phrase because ... well, now it just seems mean. She's right. It ain't a great look.

The backlash was immediate, and even though plus-size model Tess Holiday seemed to laugh it off -- Revolve pulled the ad featuring the skinny white model ... then started the damage control. It says it's donating $20k to a young women's charity.

By the way, the sweatshirt cost $168 and had already sold out by the time the ad was pulled.

We haven't seen a clothing retail PR disaster this big since way back in early 2018 ... when H&M pulled this racist "monkey" business.