Chapter Text
It wasn’t actually Katsuki or Izuku who came up with the idea of a ‘secret language' but rather a friend they used to hang out with before they all went to school. In their three-year-old minds, it was a cool spy language when in actuality it was just calling playground equipment by different names. It did progress slightly but with their friend going to a different primary school than them and Izuku's quirkless diagnosis a secret language fell into the wayside pretty quickly.
However, Inko and Mitsuki were still close friends despite their sons not talking and often had catch-ups in each others house, during which their boys were left to entertain themselves as their mother’s (wrongly let’s be honest) assumed that at eight they were old enough to keep themselves busy for a few hours without causing too much chaos. Katsuki didn’t deal well with silence (as his longest maybe-friend, Deku would say that Katsuki didn’t deal well full stop but that’s a different conversation for another day perhaps.) and after a few ‘catch ups’ spent in silence in the living room of whichever family was hosting he had reached his limit.
It had been a passing comment about the stupid names they had for the swings and suddenly they were working to get the dead language revived – but better and more complex. These conversations were the only times they could talk without starting an argument, they were just Zukie and Kacchan again - getting ready to take the world by storm. By ten they had a fully-fledged sign language based very slightly on JSL signs and what little they knew of the Hero and Military sign languages. When they’ve done they go back to stilted silence in living rooms and this time its Iizuku who breaks the tension, they might not be able to see each other's hands they should make a morse code version and so, suddenly, they’re both learning normal morse code – neither will admit it but they both learn it during their downtime so they can show off to the other when their parents meet up. Taking turns to teach each other the letters they'd learnt since last time. Then they’re adapting it to their own language (if they use it to talk shit about their teachers then that’s no one’s business but their own (and if Izuku smiles a bit more now that Kacchan will be civil with him outside of living rooms and the dojo then no one but Katsuki knows why))– finally in the year before they both try for UA they make a vocal element, essentially it’s just morse code with tongue clicks but they use a short hum as a question mark and a low pitched whistle for an exclamation mark with a short high pitched whistle before a sentence to show it as a warning or of importance.
But then suddenly they are at UA and Izuku has a quirk and when he taps out Eraserhead's hero stats he gets nothing back. At the end of their first day, he tries to tell Kacchan the truth but the other boy walks away. He lasts a week. Izuku spends an entire lesson tapping out in painstaking detail how he met all might – if Kacchan tenses at the part where Izuku was on a roof by himself then he won't mention it (he’d already seen how much the blond had hovered for weeks and weeks, kept up constant streams of taps to try and keep Izuku out of his head after he made that stupid remark, had seen the dark circles the boy wore when sleep evaded him and knew that Kacchan had punished himself for it than Izuku ever could) – how he came to know about the man’s condition, how he got his quirk and his training that he spent all his summer on. he misses the whole lesson but resolves to get the missed lecture from Todoroki or Iida who always have meticulous notes. As the bell goes he hears a tap back. Don’t lie. And hidden to all but one in the chaos of everyone leaving Izuku whistles a high pitched note before clicking his tongue. I'm not. He leaves, he's done his part in trying to keep their friendship, comradeship, whatever it was that they had. The next day when Izuku taps out a remark about how if they could bottle Present Mic’s energy they could run japan forever out of habit he’s taken by surprise a bit that he actually gets a response commenting on the teachers, quite frankly, ridiculous hairstyle. He risks a glance at the explosive ash blond to see him resolutely at his notes but thinks there is a smirk playing at his lips and his pen is twirling around his fingers rather than scribbling in his notebook.
It becomes a thing.
When they are coving content the two of them have already studied, they spend the lesson tapping out comments about their peers and teachers – normally Bakugou talking shit about the rest of the Bakusquad (how he makes morse code sound fond Midoriya will never know) and Izuku talks about his friends and the stats of their Pro Hero teachers.
Shitty Hair dyed his hair in Tape Face’s room and now it fucking stinks.
Eraserhead had a battle recently so that’s why he’s not using his quirk on us.
Pikachu thinks they’re being subtle but they keep staring out the window I think they forgot to take their meds again.
You can set your phone to send text messages at a certain time – you could do it to text them in the morning.
I don’t need your help. Half and half was staring at your All Might figurine when you showed the class last week.
Great thank you Kacchan. His birthday is soon and I had no idea what to get him.
Whatever. did you see Mirko's latest fight?
Yeah, she has a new move when she kicked off of the roof.
Nah that's because she twisted her knee in the fight before last, remember.
Oh yeah, well Hawks definitely has a new technique.
Yeah, his turns are much sharper now, did you see his team up with Present Mic?
Yeah, I wouldn't have paired them together but I got loads of notes like -
- okay let's not tap out your whole notebook, do you think Present Mic styles his hair like that?
I fucking hope not. Do you think support has to give him the hair gel if it falls under his hero costume?
Wait, would they??
“Midoriya, Bakugou I know study periods aren’t the most entertaining but could you both stop tapping you are distracting your classmates.”
Izuku blushed as he stammered apologies at Thirteen whilst Katsuki simply rolled his eyes and continued with the maths worksheet he had been filling in between tap-talking with Deku.