What does .delegate mean in groovy?

The delegate of a closure is an object that is used to resolve references that cannot be resolved within the body of the closure itself. If your example was written like this instead:

def say = {
  def m = 'hello'
  println m
}
say.delegate = [m:2]
say()

It prints ‘hello’, because m can be resolved within the closure. However, when m is not defined within the closure,

def say = {
  println m
}
say.delegate = [m:2]
say()

the delegate is used to resolve the reference, and in this case the delegate is a Map that maps m to 2.