side + line

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1. an activity done in addition to one's main job, especially to earn extra income:

副業, サイドビジネス

e.g. as modifier : a sideline career as a stand-up comic.

an auxiliary line of goods or business:

(商店の)専門外取り扱い品

e.g. electronic handbooks are a lucrative sideline for the firm.

2. (usually sidelines) either of the two lines bounding the longer sides of a football field, basketball court, tennis court, or similar playing area.

(競技のフィールドコートの)側線, サイドライン

the area immediately outside either sideline as a place for nonplayers, substitutes, or spectators:

〖～s〗 サイドラインの外側〘控え選手などの位置〙

e.g. his son watched from the sidelines. See also on the sidelines below.

cause (a player) to be unable to play in a team or game:

(けがなどで)出場メンバーからはずされる

e.g. he has been sidelined for the last six weeks with a fractured wrist.

remove from the center of activity or attention; place in a less influential position:

(重要なポストから)はずされる, 脇に追いやられる.

e.g. a respected lawyer will be sidelined by alcohol abuse.

in (or from) a position where one is observing a situation but is unable or unwilling to be directly involved in it.