1. (verb)collapse, fall in, cave in, give, give way, break, founder break down, literally or metaphorically
"The wall collapsed"; "The business collapsed"; "The dam broke"; "The roof collapsed"; "The wall gave in"; "The roof finally gave under the weight of the ice"
2. (verb)fall in to take one's place in a military formation or line
"Troops fall in!"
3. (verb)join, fall in, get together become part of; become a member of a group or organization
"He joined the Communist Party as a young man"