妹妹
mèimei · HSK 1
sister
👧 妹妹 = 妹 + 妹 (younger sister, doubled for affection)
The character 妹 combines 女 (woman) with 未 (not yet), suggesting a not-yet-grown female — a younger sister. Doubling 妹妹 is the casual, affectionate form, common in Mandarin for family terms.