The Collaborative International Dictionary of English v.0.48:

Nethinim \Neth"i*nim\, n. pl. [Heb., pl. of n[=a]th[imac]n
   given, granted, a slave of the temple, fr. n[=a]than to
   give.] (jewish Antiq.)
   Servants of the priests and Levites in the menial services
   about the tabernacle and temple.
   [1913 Webster]

Easton's 1897 Bible Dictionary:

Nethinim
   the name given to the hereditary temple servants in all the
   post-Exilian books of Scripture. The word means given, i.e.,
   "those set apart", viz., to the menial work of the sanctuary for
   the Levites. The name occurs seventeen times, and in each case
   in the Authorized Version incorrectly terminates in "s",
   "Nethinims;" in the Revised Version, correctly without the "s"
   (Ezra 2:70; 7:7, 24; 8:20, etc.). The tradition is that the
   Gibeonites (Josh. 9:27) were the original caste, afterwards
   called Nethinim. Their numbers were added to afterwards from
   captives taken in battle; and they were formally given by David
   to the Levites (Ezra 8:20), and so were called Nethinim, i.e.,
   the given ones, given to the Levites to be their servants. Only
   612 Nethinim returned from Babylon (Ezra 2:58; 8:20). They were
   under the control of a chief from among themselves (2:43; Neh.
   7:46). No reference to them appears in the New Testament,
   because it is probable that they became merged in the general
   body of the Jewish people.