適性
てきせい · tekisei · JLPT N1 · noun
aptitude
🎯 適 = suitable + 性 = nature/trait: a suitable nature, i.e. aptitude
適 (fit, suitable) + 性 (nature, quality). Compare 適職 (a job that fits you): tekisei is the natural fit between a person and a task.
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.'