性
せい · sei · JLPT N3 · noun
sexual activity
🧬 性 = heart 忄 + born 生: the nature you are born with, hence sex or gender
The left part 忄 is the heart radical and the right part 生 means life or birth. Together they first meant innate nature, a sense still found in 性格 (せいかく, personality). From there it came to mean sex and gender. 生 also gives the reading: 生 is せい, and so is 性.
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.'