okay so you know the whole thing about how TsukkiYama is canon?? well, let me share a thought -
what if Yams calls Tsukishima “Tsukki” because the first time it happened, it wasn’t intentional. What i’m saying is, let’s imagine that Yamaguchi finally got around to confessing his feelings to Tsukishima, and Tsukishima was silent bc “wow this boy likes me too wOW” and then Yams goes to apologise for being so sudden but he’s cut off so instead of saying “Sorry, tsukishima,” it becomes “sorry, tsukki-” and he’s cut off bc tsukki kissed him
And this is why Yams always says “Sorry, Tsukki!” loudly because it’s like… how everything started. How their relationship started, how ‘tsukki’ started.