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I would like to make my contribution to the Chuuya has reading glasses headcanon:
Chuuya calls Ango "Doc glasses." This started when he just got reading glasses, and was insensitive about needing them.
But one day, there's a joined meeting between the division and Ango sees Chuuya with reading glasses—
He opens his mouth to speak. Chuuya sees him, realizes the situation, and before Ango can attempt to tell him that he no longer has the right to call him "Doc glasses," Chuuya very calmly looks down at what he's reading, glasses remaining, and says, "if you make one single comment, then you won't need glasses anymore." He looked Ango in the eyes. "Ever again. Got it, Doc Glasses?"
Ango has enough self-preservation to agree. And thus, the nickname remained.