Meaning: a younger sister
I’m gonna get into this since most people have encountered this word at least once since being exposed to Japanese media. The word 妹 pertains to a person’s “younger sister,” as they use different words to address their family members. Use this word if you’re talking about YOUR younger sister, but if you’re talking about another person’s, be polite and use “妹さん” instead (adding -san gives a better impression that you’re giving respect. Doing so without it indicates either closeness, or just being plain rude).
女 (woman, female) + 未(not yet) = 妹(imouto)
This word has two characters, 女(おんな / onna), pertaining to the female gender and 未(み / mi), which can also mean “not yet” (also, the same kanji used in 未だ/まだ). “Woman not yet” is what perfectly describes a young female member of the family, thus, the word.
Sample phrases / sentences:
Watashi no imouto ga Manila ni imasu.
My younger sister is in Manila.
Watashi no kazoku wa gonin desu. Chichi to haha to imouto ga futari imasu.
There are five people in my family. I have a father, a mother, and two younger sisters.
Kono naka de hitori ga imouto ni iru!
One of them is my younger sister!
Ore no imouto ga konna ni kawaii wake ga nai!
My little sister can’t be this cute!