When your brother, a Hebrew man or a Hebrew woman, is sold to you, and serves you for six years, then in the seventh year you shall let him go free from you. Deut 15:12.

Here the expressions 'a Hebrew men' and 'a Hebrew women' are used because servitude is the subject. In Jeremiah,

At the end of seven years you shall let go every man his brother that is a Hebrew who has been sold to you and has served you for six years. Jer 34:9, 14.

Here similarly the name 'Hebrew' occurs because servitude is the subject, though the sons of Jacob are not called Hebrews in other places in the Prophets. In Samuel,

The Philistines said, Take heart and acquit yourselves like men, lest you be slaves to the Hebrews as they have served you. 1 Sam 4:9.

Here the meaning is similar.

[2] In Moses,

Jehovah said to Moses, Go in to Pharaoh and say to him, Thus said Jehovah the God of the Hebrews, Let My people go so that they may serve Me. Exod 9:1, 13; 10:3.

Here too because of their servitude they were called Hebrews. Potiphar's wife referred to Joseph as a Hebrew,

She called to the men of her house, and said to them, See, he has brought us a Hebrew man to make sport of us. Gen 39:14.

He is called 'a Hebrew' because he was a slave there. The chief of the cup-bearers said to Pharaoh,

There was with us a Hebrew lad, a slave of the chief of the guards, and he interpreted our dreams to us. Gen 41:12.

In addition the Egyptians called the children of Israel Hebrews, because they were slaves, or in slavery, as is well known from Exod 1:15, 16, 19, and elsewhere.