Hc that instead of merlin's beard/Godric Gryffindor Sirius starts using variations of Lupin's name as exclamations in his everyday language like
"Lupin's furry tail, that was close"
"By Lupin's name, I will hex you"
"Remoony's baggy Y-fronts, Minnie's going to kill us!!"
"Lupin's most luxurious pelt, those dungbombs did a number, huh?"
"Moony's pants, what have we done"
It drives Remus up the wall but it catches up with James, Pete and the girls and by extension, the whole Gryffindor tower and soon there are several variations going around and almost on every corner you can hear someone go
"Acting the Lupin"
"Buggering Lupin"
"Gobsmacked Remus"
"In the name of Lupin"
"By the power invested in me by Remus Lupin"
It's hilarious and by the end of their time at Hogwarts, Remus doesn't even notice it anymore to the extent that it goes from blushing profusely and dropping whatever he's holding in surprise to blushing mildly and stuttering.