fall out

If a tooth or your hair falls out, it becomes loose and separates from your mouth or head:
Her baby teeth are starting to fall out.
A side effect of the treatment is that your hair starts to fall out.

informal
to argue with someone and stop being friendly with them:
He left home after falling out with his parents.
She'd fallen out with her boyfriend over his ex-girlfriend.

If soldiers fall out, they move out of a line:
"Fall out, men!" shouted the sergeant-major.
(Cambridge Advanced Learner's Dictionary & Thesaurus からの fall out の定義 © Cambridge University Press)

アメリカ英語辞典
(of an object) to drop from a place where it was attached or contained:
A few pages fell out of the book.

to have an argument or disagreement that ends a relationship:
The two fell out over coaching tactics a long time ago.
(Cambridge Academic Content Dictionary からの fall out の定義 © Cambridge University Press)

- Cambridge Dictionary