What is Linesman ?
Linesman is (noun) 1. a man who looks after electric, telephone or railway lines The linesmen struggled for days to restore power to the areas that had been cut off by the storm. (NOTE: The US term is lineman.) 2. an official who stays on the side of the playing area in a game to see if the ball goes over the line, and helps the referee to spot any breaking of the rules After consulting with his linesman, the referee awarded a free kick. The linesman flagged that the ball had gone out of play. (NOTE: The plural is linesmen.)
source: Easier English, Student Dictionary Upper Intermediate Level