Boyd’s Bible Dictionary:

(dedicated). Assistant priests. A class, or order, associated with the temple service and wardship (1 Chron. 9:2; Ezra 7:24; 8:17-20).

Concise Bible Dictionary:

Name, signifying “given, devoted ones,” applied to those who assisted the Levites in the service of the tabernacle and the temple. The name does not occur until 1 Chronicles 9:2, and afterward in Ezra and Nehemiah. The Gibeonites were made “hewers of wood and drawers of water for the congregation and for the altar” (Josh. 9:27). These are not mentioned after 2 Samuel, so that they and their descendants may have been the “Nethinim.” It does not appear that God appointed them, as He did the Levites, but “David and the princes” appointed them “for the service of the Levites” (Ezra 8:20). Some of the Midianite captives were also given to the Levites (Num. 31:46-47). After the return from exile the Nethinim are called the “ministers of this house of God.” They were, along with the priests and Levites, exempt from “toll, tribute, or custom” (Ezra 7:24). A list of them is given in Ezra 2:43-54; Nehemiah 7: 46-56,60; Nehemiah 10:28.

Jackson’s Dictionary of Scripture Proper Names:

given ones

From Manners and Customs of the Bible:

Ezra 2:43. The Nethinims.
These were men who assisted the Levites in performing the meanest offices connected with the temple service. Part of them lived in Jerusalem, and part were distributed among the Levitical cities. They are supposed to have been Canaanites reduced to servitude (Josh. 9:21-27) and captives taken in war, who were set apart to this service, and therefore called nethinim: the given, the devoted. They were held in low esteem by the Jews, occupying a social position even lower than the rammer or illegitimate offspring.