1. To take one's place in a military formation or line
   "Troops fall in!"
    
2. Become part of; become a member of a group or organization
   "He fell in with the local theatre group";
   - join
    
3. Break down, literally or metaphorically
   "The wall fell in";
   - collapse, cave in, give, give way, break, founder

Derived forms: fallen in, falls in, falling in, fell in

Type of: act, change, fill, move, occupy, take