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When Bucky is upset he curses in Russian. It rolls off his tongue and between his lips so easily that Steve is fairly sure he isn't aware he's doing it but that doesn't make it any less terrifying half the time.
The first time he does it, he's in Sam's kitchen cooking them all breakfast. Sam, Steve, and Natasha are sitting on the sofa involved in friendly bickering over the taste of Coca-cola when they hear a loud crash followed by a very threatening sounding "Suka sranaya" ground out through clenched teeth. Natasha just laughs and laughs and laughs while the two boys stare at her with looks similar to lost puppies.
After the initial time Steve finds that Bucky swears allot when he's upset or distressed. Its almost always in Russian and if Natasha is around she almost always laughs.
He stubs his toe on the coffee table a few days later and yells at the coffee table "Shluha vokzal'naja" Steve stares at him like a deer in headlights and Natasha snickers from across the room making Bucky turn pink.
One day, out of sheer curiosity Steve sits down and googles something Bucky had mumbled at Tony afew days before that made Natasha laugh so hard she had to leave the room and he turns beet red.
"Uhh Bucky?" He nibbles his lip thoughtfully, glancing over to the ex assassin sitting on the couch. "Did you tell Stark to-" He clears his throat, "get a horse to fuck him?"
Bucky looks up from his book with the most innocent look on his face. "Yeah." He says it so matter-of-factly that Steve cant help but burst out laughing and Bucky has that confused look on his face.
Steve just shakes his head and goes to sit on the sofa beside Bucky all grin and kisses his forehead gently. So that's why Natasha kept laughing.