A male deprived of the testes or external genitals. Such were excluded from serving as priests (Leviticus 21:20 ) and from membership in the congregation of Israel (Deuteronomy 23:1 ). Eunuchs were regarded as especially trustworthy in the Ancient Near East and thus were frequently employed in royal service. By extension, the Hebrew word translated eunuch could be used of any court official (At Genesis 37:36 and Genesis 39:1 the reference is to a married man). The Greek term translated eunuch is literally one in charge of a bed, a reference to the practice of using eunuchs as keepers of harems (Esther 2:3, 2:6, 2:15). Part of Isaiah's vision of the messianic era was a picture of the eunuch no longer complaining of being “a dry tree”, one without hope of descendants, because God would reward the faithful eunuch with a lasting monument and name in the Temple which would be far better than sons or daughters (Isaiah 56:4-5). Ethiopian eunuch of Acts 8:27 was reading from Isaiah's scroll.
A “eunuch for the sake of the kingdom of heaven” (Matthew 19:12) is likely a metaphor for one choosing single life in order to be more useful in kingdom work. Compare 1 Corinthians 7:32-34.