to feel that you belong to a particular group and are accepted by that group:
It's no surprise she's leaving - she never really fitted in.
If one thing fits in with another thing, they look pleasant together or are suitable for each other:
It's a very nice sofa but it doesn't fit in with the rest of the room.
(Cambridge Advanced Learner's Dictionary & Thesaurus からの fit in の定義 © Cambridge University Press)
to belong with other things, or to be accepted by other people in a group:
This chair doesn’t fit in with our furniture.
She fit in well at school.
(Cambridge Academic Content Dictionary からの fit in の定義 © Cambridge University Press)
to feel you belong to a particular group and are accepted by the other members:
Potential board members need to understand that they must fit in.
fit in with sb How do you think she will fit in with the rest of the staff?
to be suitable or right in a particular situation:
fit in with sth Industry observers believe a sell-off would fit in with the company's new strategy.
(Cambridge ビジネス英語辞典 からの fit in の定義 © Cambridge University Press)
- Cambridge Dictionary