The term sniper was first used in 1824 in the sense of the word “sharpshooter”. The verb “to snipe” originated in the 1770s among soldiers in British India where a hunter skilled enough to kill the elusive snipe (a wading bird) was dubbed a “sniper”.
During the American Civil War, the common term used in the United States was “skirmisher”. Throughout history armies have used skirmishers to break up enemy formations and to thwart the enemy from flanking the main body of their attack force. They were deployed individually on the extremes of the moving army primarily to scout for the possibility of an enemy ambush. Consequently, a “skirmish” denotes a clash of small scope between these forces. In general, a skirmish was a limited combat, involving troops other than those of the main body. The term “sniper” was not in widespread use in the United States until after the American Civil War. (Wikipedia)