A.T. Robertson Commentary Galatians 4:22

"For it is written, that Abraham had two sons, one by the handmaid, and one by the freewoman." — Galatians 4:22 (ASV)

By the handmaid (εκ της παιδισκης). From Ge 16:1. Feminine diminutive of παις, boy or slave. Common word for damsel which came to be used for female slave or maidservant (Luke 12:45) or doorkeeper like Mt 26:29. So in the papyri.