side·line / ˈsīdˌlīn/ • n. 1. an activity done in addition to one's main job, esp. to earn extra income: [as adj.] a sideline career as a stand-up comic. ∎ an auxiliary line of goods or business: electronic handbooks are a lucrative sideline for the firm.2. (usu. sidelines) either of the two lines bounding the longer sides of a football field, basketball court, tennis court, or similar playing area. ∎ the area immediately outside such lines as a place for nonplayers, substitutes, or spectators: his son watched from the sidelines. See also on the sidelines below.• v. [tr.] (often be sidelined) cause (a player) to be unable to play on a team or in a game: he has been sidelined for the last six weeks with a fractured wrist. ∎ fig. remove from the center of activity or attention; place in a less influential position: a respected lawyer will be sidelined by alcohol abuse.PHRASES: on (or from) the sidelines in (or from) a position where one is observing a situation but is unable or unwilling to be directly involved in it.