Nethinim was the term applied to those who were set apart to do the menial work of the sanctuary. Their function appears to have begun with the Gibeonites who served as hewers of wood and carriers of water in the sanctuary (Joshua 9:21). By some writers the Gibeonites become the original Nethinim, while others hold that the Nethinim performed the same functions as the Gibeonites, only with a slight variance; where the Gibeonites hewed wood, the Nethinim hewed stone. The initial reasons for the development of the Nethinim was the persecution of the Gibeonites by Saul, their numbers were decreasing, and the massacre at Nob (1 Samuel 22:1-19), and then David required more servants to perform menial tasks. David and other leaders gave the Nethinim to the Levites as servants (Ezra 8:20). Probably some were prisoners of war, who became proselytes, and were called Nethinim in post-exilian times (1 Chronicles 9:2; Ezra 2:43; Nehemiah 7:46). No prescribed duties assigned to the Nethinim are described in scripture because this assignment was left entirely to the discretion of the Levites.
The number of Nethinim totaled between 1200 to 1400 people when returning from captivity. Since they were controlled by the Levites, and were solely to perform the menial services, hewing wood, carrying water, helping with the rebuilding and repairs of the tabernacle, obviously some lived close to the sanctuary, while others, as before the exile, dwelt in different Levitical cities (Ezra 2:70). They were governed by a chief of their own body (Ezra 2:43; Nehemiah 7:46). Along with other sacred ministers they were exempted from taxation by the Persian satrap (Ezra 7:24), and were supported by the temple treasury and second tithes.
The Nethinim were considered to be of a very low class or position in Jewish society; they were even considered below the Mamzer, illegal offspring. According to Jewish authorities they were restricted to intermarriage among themselves, and if a Jew or Jewess married one of them, the issue (child) shared in all the disqualifications of the Nethinim; and they were not exempted from military service when newly married. If a woman was suspected of being deflowered, or if she had an illegitimate child, it was ascribed to a Nathin, unless the mother could show proof of the fatherhood. The decision of a court of justice was invalid if one of the members was a Nathin, as he was not considered to be a member of the congregation as specified in Leviticus 4:13; Numbers 35:24. Eventually, it appears, they were emerged into the Jewish population, as no allusion to them occurs either in the Apocrypha or the New Testament. A.G.H.
Unger, Merrill F., Unger’s Bible Dictionary, Chicago, Moody Press, 1966, pp. 790-791