Peter getting a 'daddy' tattoo on his hip. And didnt tell Tony at all about it, but Bucky did take Peter to get it.
Imagine, Tony catching a glimpse of the bandage because Peter bumps into something in the lab, and he’s immediately concerned, because Peter usually heals quick enough not to need bandages and the like. He opens his mouth to ask but then maybe he gets a phone call or someone comes into the lab, or something equally distracting, and he loses his chance.
Then he forgets, and isn’t reminded about it until a similar moment has Peter’s shirt lifting up, and in place of the white he saw covering the skin last time, Tony catches a small, black, y. Before he has a chance to ask the first of the many questions that pop into his head, Peter’s shooting him a bright smile and a sweet, “bye, Mr. Stark,” twirling out of the lab and into the elevator.
It’s hours later when Tony processes that what he glimpsed was innocent Peter Parker’s first (?) tattoo. He spends an equivalent amount of time trying to figure out what it could be, racking his brain for why Peter could possibly have had a y etched onto his body. He reckons he must just be sleep deprived when he realizes how glaringly obvious the answer is - the tattoo must be for May. He finally goes to sleep, relentless curiosity satiated.
But then he’s heading into the kitchen the next day, originally having told Peter he didn’t want anything then changing his mind, when he hears Peter chattering away with somebody. It only takes a few seconds to place Barnes’s deep baritone, and he’s about to head in, trusting he’s not interrupting a deep conversation when he hears Bucky ask “can I see it?”
The idea hits him so fast he doesn’t understand why it didn’t occur to him before, genius that he is, and he’s suddenly struggling to hold back his lunch as he considers the fact the tattoo says Bucky instead of May, and he doesn’t know why the thought has him blinking back tears. (It’s a lie, he knows exactly why, because he spent most of yesterday convincing himself the boy hadn’t etched Tony onto his body in an act of devoted hero worship, and he had spent even longer convincing himself that was a good thing, that he didn’t want a permanent reminder of him on Peter’s otherwise pale, unblemished, perfect body.) Now the thought that Peter, his Peter, has another man’s name permanently inked onto his skin, makes Tony want to bury himself under liters of whatever alcohol will help himself forget the fastest.
It turns out to be whatever alien alcohol he’s pilfered from Thor’s bedroom, and he’s quickly nearing the bottom of his second (third?) bottle. He was smart enough to cut off Peter’s access to the floor, had FRIDAY relay the message that Peter’s ‘security clearance is, unfortunately, not high enough for him to be privy to what Sir will spend the rest of the night working on, but he is, as always, more than welcome to return tomorrow.’ He feels a brief pang of regret at the forlorn expression he catches on Peter’s face as he rides the elevator down, but he quickly drowns that with his third (fourth?) bottle.
Peter lets Tony wallow for exactly three days before he’s overriding FRIDAY’s commands, (with the AI’s help) and sneaking into the lab. His super strength makes it quite easy for him to wring the bottle (whichever number it may be) from Tony’s grip, wrestle him into the shower before depositing him in his bed, tucking him under the covers for some much needed rest.
When Tony wakes Peter convinces him to have a conversation like a grown adult, complete with an argument, slightly (very) raised voices, and a fair amount of emotional blackmail.
They spend the rest of the day in bed, alternating between Tony worshipping the Daddy inked across Peter’s hip, and wringing the word from his pretty pink lips in all its variations, from gasps and groans to moans, whimpers and wails.