an activity that you do as well as your main job or business, in order to earn more money
as a sideline
Zoe does a bit of freelance photography as a sideline.
not taking part in an activity even though you want to or should do
stand/stay/remain etc on the sidelines
You can't stay on the sidelines for ever; it's time you got involved.
the area just outside the lines that form the edge of a sports field
on the sidelines
Wenger stood on the sidelines shouting instructions.
a line at the side of a sports field, which shows where the players are allowed to play