Left-side woman radical - the indexing radical. In archaic China the earliest surnames were matrilineal, passed through the mother’s line; many of the oldest surnames preserve 女, as in 姜, 姬, and 姚. The word 姓 itself means "born of a woman".
Right-side 生 supplies the sound, shifting to xìng in 姓. It doubles as semantic: "born" joined to "woman" = a surname inherited via maternal birth. Phonetic family: 性, 星, 牲.
