a line at the side of something.
a business or activity pursued in addition to one's primary business; a second occupation.
an additional or auxiliary line of goods: a grocery store with a sideline of household furnishings.
either of the two lines defining the side boundaries of a field or court.
sidelines, the area immediately beyond either sideline, where the substitute players sit.
sidelines, the position or point of view taken by a person who observes an activity or situation but does not directly participate in it.
verb (used with object), sidelined, sidelining.
to render incapable of participation, especially in anything involving vigorous, physical action, as a sport: An injury to his throwing arm sidelined the quarterback for two weeks.

1685–95, Americanism; side1 + line1 Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
sideline (ˈsaɪdˌlaɪn)
1.  sport a line that marks the side boundary of a playing area
2.  a subsidiary interest or source of income
3.  an auxiliary business activity or line of merchandise
4.  to prevent (a player) from taking part in a game
5.  to prevent (a person) from pursuing a particular activity, operation, career, etc

Collins English Dictionary - Complete & Unabridged 10th Edition
2009 © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009
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Word Origin & History

"lines marking the limits of playing area" (on a football field, etc.), 1862, from side (adj.) + line (q.v.). The fig. sense of "position removed from active participation" is attested from 1934 (because players who are not in the game stand along
the sidelines). The verb meaning "put out of play" is from 1945.
Online Etymology Dictionary, © 2010 Douglas Harper
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Example sentences
Beekeeping can be a fascinating hobby, a profitable sideline, or a full-time
It was there on the sideline, and the spoken word movement brought poetry back
  to the people.
And you do your creative work as a sideline, a second job, or a hobby.
Their financial reporting business is only a sideline.
Image for sideline
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