The story
Calque from Japanese 亀(きっ)甲(こう)括(かっ)弧(こ) (kikkōkakko, literally “tortoise shell bracket”), due to their resemblance to the shell of a tortoise. By surface analysis, tortoise shell + bracket.
Etymology from Wiktionary, CC BY-SA 4.0.
tortoise shell bracket
Today it means: Either of a pair of symbols,〔 〕, sometimes used in Japanese to enclose comments in quoted text. Full definition →