I. ˈ ̷ ̷ˌ ̷ ̷ noun
1. : a line extending along or marking the side of something ; especially : a line running usually at right angles to a goal line or end line and marking a side of an area (as a court or field of play) used for sports — see field hockey illustration
2.
a. : a line of goods sold in addition to one's principal articles of trade
b.
(1) : a course of business or activity pursued aside from one's regular occupation
a farmer who had taken to writing as a sideline — A.J.P.Taylor
(2) : something subsidiary to the principal subject considered
his pursuit … took him into many interesting sidelines — Current Biography
3.
a. : the space immediately outside the lines along either side of an athletic field or court — usually used in plural
b. : the standpoint of persons not immediately participating (as in an athletic contest) — usually used in plural
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- on the sidelines
II. transitive verb
Etymology: sideline (I)
1. West : to restrain or hobble (an animal) by tying together the front and hind leg on the same side of the body
2. : to prevent (as a player) from taking part in a game or other activity
a sore shoulder sidelined him
has been sidelined with a broken arm