Elhanan. The brother of Goliath the Gittite: the relative word brother is not in the Hebrew text, but is fitly supplied out of the parallel place, 1 Chronicles 20:5, where it is expressed. And such defects of relatives are not unusual in Scripture. Thus the word wife is understood, Matthew 1:6 1 Thessalonians 19:25; and father or mother, Mark 15:40,47, compared with Mark 16:1 Luke 24:10; and son, Matthew 4:21 Mark 2:14 1 Thessalonians 21:15; and brother, Luke 6:16, compared with Judges 1:1. And such ellipses do also frequently occur in profane authors. Although the place may be and is otherwise rendered, Elhanan, the son of Jaare-oregiro, slew Beth-halachmi, or Lahmi, (as he is called by way of abbreviation, 1 Chronicles 20:5, which is very frequent in the Hebrew tongue,) who was (which words are frequently understood in the Hebrew text) with (so eth is oft rendered, as hath been noted before) Goliath the Gittite, i.e. in his company, bred up with him to the war, and related to him as his brother. Or, he slew Beth-halachmi, a Goliath (or another Goliath) of Gath, or the Gittite. So the name of the giant was Beth-halachmi, who may be here called Goliath, not only for his near relation to him, being his brother, but for his exact resemblance of him in feature, or in stature and strength, or in courage and military skill; as John the Baptist was called Elias for the like reason. Peradventure also, after the death of the first and famous Goliath the Gittite, 1Sa 17, that name was either given to him by others, or taken by himself. Like a weaver's beam, in thickness. See 1 Samuel 17:7.
Related Commentaries of 2 Samuel 21:19
2 Samuel 21:19
19 And there was again a battle in Gob with the Philistines, where Elhanan the son of Jaareoregim,g a Bethlehemite, slew the brother of Goliath the Gittite, the staff of whose spear was like a weaver's beam.