Okay, THIS one is gonna take some explaining. This is, I suppose, a gift/tribute pic in honor of Bookish Delight's awesome story, "Blunder and Lightning" Go read it!
If you haven't read it, then without giving too much away, here's an excerpt.
[Excerpt from "Blunder and Lightning," Chapter Three]
She held up an outfit. Rainbow Dash held back her lunch.
"No! Anything but that!"
Said outfit was full-length, black with white frills... and had an apron as its centerpiece.
"I need an extra hoof to help cater. Someone who can handle a giant-sized hungry Apple Clan with speed n' efficiency! Figure it may as well also be somepony who'd look simply scrumptious in this..."
"F-flattery is not going to work here, Applejack," Rainbow Dash said, fighting a blush. "Honestly, what sick twisted mare gave you this idea?"
"Fluttershy, actually, when she gave you that apron for your birthday! So a couple days ago, I asked Rarity to make one of these for you. She was plum tickled enough by the idea to do it--busted this out in record time, even! I ain't never seen that gal work so fast! Like she was downright possessed!" She winked. "So, how'd your apologizin' to her go?"
Rainbow Dash backed away, now understanding everything. The softballing Rarity had given her... it'd been because she'd known about this in advance. She and Applejack were in on this together. "You knew. You knew I'd be back to apologize!"
"'Course we did. Like I said, you're a normal pony like the rest of us--and I know you have a kind heart. It was just a matter of time. Check and mate, as Twilight would say."
Rainbow Dash sighed in defeat, and grabbed the outfit. "I will remember this so hard."
--END OF EXCERPT--
This scene takes place after the story ends at the Apple Family Reunion where Rainbow goes to take Braeburn's order...and I'm guessing ended up taking his breath away. I don't know who AJ is laughing at here, Dash or her poor cousin. I'm betting on the latter. Rainbow's clearly embarrassed but I don't think she's minding the effect on Braeburn too much though.
Art by Trotsworth [link]