die Seitenlinie
sideline
In sports, the line that marks the side boundary of a playing field.
Der Ball rollte über die Seitenlinie ins Aus.
The ball rolled over the sideline and out of bounds.

sideline
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sideline
In sports, the line that marks the side boundary of a playing field.
Der Ball rollte über die Seitenlinie ins Aus.
The ball rolled over the sideline and out of bounds.

The word is clearly composed of 'die Seite' (the side) and 'die Linie' (the line). So it literally describes a line located at the side of something.
Der Schiedsrichter entschied, dass der Ball die Seitenlinie nicht vollständig überquert hatte.
The referee decided that the ball had not completely crossed the sideline.
The figurative meaning comes directly from sports. Like a coach or substitute player standing on the sideline, you stand outside and observe without actively taking part in the 'game'—for example in politics or business.
Er musste das Projekt von der Seitenlinie aus beobachten, nachdem man ihm die Leitung entzogen hatte.
He had to watch the project from the sidelines after he was stripped of leadership.
In a sports context, the prepositions 'an' (at/on, position) and 'über' (over, movement) are typical. You stand 'an der Seitenlinie' (on the sideline), or the ball rolls 'über die Seitenlinie' (over the sideline).
Der verletzte Spieler stand nervös an der Seitenlinie.
The injured player stood nervously on the sideline.
In the figurative meaning, the preposition 'von' (from) is often used. Typical phrases are 'von der Seitenlinie zusehen' (to watch from the sidelines), 'etwas von der Seitenlinie kommentieren' (to comment on something from the sidelines), or 'an die Seitenlinie gedrängt werden' (to be pushed to the sidelines).
Als ehemaliger Chef kann er die Entscheidungen nur noch von der Seitenlinie kritisieren.
As the former boss, he can only criticize the decisions from the sidelines now.