添加
tiān jiā · HSK 7
to add
➕ 添加 = 添 (pour in more) + 加 (plus) — two add-characters stacked for emphasis
Both halves mean add: 添 carries the water radical 氵for topping up a liquid, and 加 is the plain plus/add character seen in 加油. Doubling them gives the firm sense of tacking something extra on.
添 — water radical 氵 + 忝 (phonetic)
添 is a phono-semantic compound. The water radical 氵 hints at the meaning 'to add water, to pour in more,' while 忝 supplies the sound. From the literal sense of topping up liquid, it broadened to mean 'to add' or 'to append' anything.
加 — force 力 + mouth 口
加 is traditionally analyzed as a compound of 力 (strength) and 口 (mouth), suggesting boasting or exaggerating one's effort with words. From this it extended to the meaning 'to add' or 'increase'. Today it is used broadly for addition, both literal and figurative.