Kummerspeck: excessive weight gained due to stress eating.
A few weeks before their wedding, much to Natasha's horror, she steps into her dress, tugs it up, and finds that it is just the tiniest bit too tight around her ribcage. Clint, when called in to inspect, tells her that it looks nice, that she looks like a ripe fruit, and since that is completely unhelpful, Natasha pushes him out of the way and dashes to the bathroom and the scale, which confirms in gleeful red digits that she has indeed put on a few pounds.
"What am I supposed to do?" she sobs tearfully a few minutes later, after Clint has so kindly unlaced her and the dress is sitting in a heap of lace and satin on the floor. "It's going to be my first wedding and I can't fit into my dress!"
Clint chooses not to say anything about her choice of words ("First wedding?" he wanted to say. "Don't you mean your only wedding?") and instead pats her on the head.
"And you!" she shouts all of a sudden, pushing away from him and looking at him as though he's taken her firstborn and eaten it in front of her. "You let me gain weight!"
"I, uh, excuse me?" he asks, befuddled as she grabs him by the front of his shirt and tugs him to standing with a strength born from rage.
"You let me eat all that cake!" she roars, and Clint is rather quite surprised that she hasn't managed to shatter the glass windows that frame the Avengers Tower. "I bet this was your plan all along! Oh, here Natasha, just try this one, you want to be sure to get the right flavour for the wedding cake. It was you, this whole time!"
Clint begins to protest, but she smacks him across the face and tells him, chest heaving, to assume his position. Her tone brooks no position for argument, and Clint just shrugs and gets on his knees while Natasha runs over to their toy chest and rummages through it with a look of half-crazed glee on her face.
Three hours later, after Clint's entire body is sore and covered with raised welts from where Natasha has flogged him, she cuddles up next to him in bed, slicked with sweat, and tells him that she's figured out a brilliant weight-loss strategy.