異性

いせい · isei · JLPT N1 · noun

sexual activity

異 (different) + 性 (sex) = 異性, the different sex: the opposite sex

性 — heart radical 忄 + 生 (life/birth)
性 is a semantic-phonetic compound. The heart radical 忄 supplies the meaning, pointing to inner feelings or innate qualities, while 生 ('birth, life') gives the sound and hints at what one is born with. It originally meant 'inborn nature' and later extended to 'character,' 'gender,' and 'sex.'

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