Removed from active participation or put on the bench; excluded from something important.
From 'side line' (a boundary line along the side of a sports field), popularized in American football language in the early 1900s. Metaphorically expanded to mean being pushed aside or kept out of action.
In football, the sideline is where substitutes wait, so being 'sidelined' meant literal benching—but the phrase became so useful that we now use it everywhere from politics to business whenever someone gets kicked out of action.
Complete word intelligence in one call. Free tier — 50 lookups/day.