(A) To quarrel or disagree with someone about something
1. Tony FELL OUT with Nick about the video game.
2. The law firm was disbanded after the partners FELL OUT.
3. They FELL OUT over some little issue.
(B) To lose one’s hair or tooth. (inseparable)
1. Her hair started FALLING OUT as a result of radiation treatment.
2. When you get old, your teeth would start to FALL OUT.
3. According to the research, your hair FALLS OUT because of too much stress.