Etymology
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sideline (n.)

also side-line, "line on the side of a fish," 1768; "lines marking the limits of playing area" (on a football field, etc.), 1862, from side (adj.) + line (q.v.). Meaning "course of business aside from one's regular occupation" is from 1890. Railway sense is from 1890. The figurative sense of "position removed from active participation" is attested from 1934 (from the railway sense or from sports, because players who are not in the game stand along the sidelines). The verb meaning "put out of play" is from 1945. Related: Sidelined; sidelining.

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Definitions of sideline
1
sideline (n.)
a line that marks the side boundary of a playing field;
Synonyms: out of bounds
sideline (n.)
an auxiliary line of merchandise;
sideline (n.)
an auxiliary activity;
Synonyms: avocation / by-line / hobby / pursuit / spare-time activity
2
sideline (v.)
remove from the center of activity or attention; place into an inferior position;
The outspoken cabinet member was sidelined by the President
From wordnet.princeton.edu