性子 has two related but distinct meanings. First, it refers to a person's innate temperament or disposition — used with adjectives like 急 (impatient) or 慢 (easygoing), and in fixed compounds like 急性子 and 慢性子. Second, it refers to the strength or potency of alcohol, medicine, or other substances. Be careful with translations: for 'temper' in the emotional sense, 脾气 (píqi) is the more common word.