A ploughboy was a boy whose job was to walk with the horse (or ox) that pulled a plough, so the animal kept going and stayed on the line. The ploughman held the plough; the boy led the beast. This is farm life from before tractors. You will meet the word in old songs, poems, and history, not in a job advert today. The spelling plough is British; American English writes plow, so plowboy. Though actually, even in Britain the job itself is gone, so the word lives on in folk song and period stories.