—n. 1. a line at the side of something.
2. a business or activity pursued in addition to one's primary business; a second occupation.
3. an additional or auxiliary line of goods: a grocery store with a sideline of household furnishings. 4. Sports. a. either of the two lines defining the side boundaries of a field or court.
b. sidelines,the area immediately beyond either sideline, where the substitute players sit.
—v.t. to render incapable of participation, esp. in anything involving vigorous, physical action, as a sport: An injury to his throwing arm sidelined the quarterback for two weeks.