甲
こう · kou · JLPT N1 · noun
rank
🐢 Turtle-shell pictograph; first of the Heavenly Stems, so it means 1st in rank
The old oracle-bone form of this character shows a shell. Ancient Chinese used 甲 as the first of the ten day-names, so it came to mean first in rank.
甲 — pictograph of a seed sprout's hard shell
甲 originally pictured the cracked shell of a seed as it sprouts, or the hard carapace/armor of a turtle or insect. From this came meanings like 'shell,' 'armor,' and 'helmet.' It was later borrowed as the first of the ten Heavenly Stems, giving it the sense of 'first' or 'A-grade.'