stand by
US /stænd baɪ/
UK /stænd baɪ/

1.
to be present while something bad is happening but not do anything to stop it
:
•
He just stood by and watched the bullying happen.
•
How could you stand by and let them treat her like that?
2.
3.
to be ready to act or to be used if needed
:
•
Emergency services are on stand by.
•
The backup generator is always on stand by in case of a power outage.