sit by phrasal verb (
see also sit)
to allow something wrong or illegal to happen without doing anything about it:
I’m not going to sit by and watch a man go to prison for something I’ve done. [TahlilGaran] Dictionary of Contemporary English ▲
sit by v. 1. To stay near; watch and care for.
The nurse was told to sit by the patient until he woke up. Mother sat by her sick baby all night. 2. To sit and watch or rest especially while others work.
Don't just sit idly by while the other children are all busy. [TahlilGaran] English Idioms Dictionary ▲