If you fall out with someone, you have an argument and stop being friendly with them. You can also say that two people fall out. She fell out with her husband Mum and I used to fall out a lot. see also fallout
If something such as a person's hair or a tooth falls out, it comes out. Her hair started falling out as a result of radiation treatment
leave (a barracks, for example) in order to take a place in a military formation, or leave a military formation; "the soldiers fell out"