I. side‧line¹ /ˈsaɪdlaɪn/ noun

1. [countable] an activity that you do as well as your main job or business, in order to earn more money
as a sideline
Zoë does a bit of freelance photography as a sideline.

2. on the sidelines 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.

3. sidelines [plural] the area just outside the lines that form the edge of a sports field
on the sidelines
Wenger stood on the sidelines shouting instructions.

4. [countable] a line at the side of a sports field, which shows where the players are allowed to play

[TahlilGaran] Dictionary of Contemporary English ▲

II. sideline² verb [T, usually passive]
if you are sidelined, you are unable to play in a sports game because you are injured, or unable to take part in an activity because you are not as good as someone else:
Owen was once again sidelined through injury.

[TahlilGaran] Dictionary of Contemporary English ▲

sideline
noun
ADJ. lucrative, nice (little), profitable, useful He decided to turn his hobby into a lucrative sideline.
VERB + SIDELINE have | develop
PREP. ~ for Cider making was a sideline for many farmers.
~ in She is developing a nice little sideline in childminding for friends.
~ to The restaurant started out as a sideline to his main business.

[TahlilGaran] Collocations Dictionary ▲