Here's how it breaks down:
* "Had been" indicates a past state.
* "All but" means almost completely or nearly.
* "Dead" refers to the state of being lifeless or inactive.
Example:
"The old tradition had been all but dead for generations, but a new group of young people has brought it back to life."
This sentence implies that the tradition was nearly extinct, but a new group has revived it.
Other ways to say "had been all but dead":
* Nearly extinct
* On the verge of extinction
* In a state of near-death
* Almost gone
* In a state of near-inactivity
Let me know if you'd like any other examples!