Excerpt from Questions.org
In Scripture, an armor-bearer (also spelled armorbearer and armor bearer) was a servant who carried additional weapons for commanders. Abimelech (Judges 9:54), Saul (1 Samuel 16:12), Jonathan (1 Samuel 14:6-17), and Joab (2 Samuel 18:15) had armor-bearers/armorbearers. Armor-bearers were also responsible for killing enemies wounded by their masters. After enemy soldiers were wounded with javelins or bows and arrows, armor-bearers finished the job with clubs and/or swords. After the time of David, armor-bearers are no longer mentioned, likely due to the fact that commanders began to fight from chariots (1 Kings 12:18; 20:33).
[…] Questions.org. Bearing Armor. http://www.gotquestions.org/armor-bearer.html