As the docs state, Ruby’s undef_method

prevents the current class from responding to calls to the named method.

This means you can do some weird things to the inheritance hierarchy. I’ll use the following code example to illustrate.

class Phone
  def ring
    puts 'brrrrriiing'
  end
end

class Smartphone < Phone
  def ring
    puts 'boop beep boop'
  end
end

class Iphone < Smartphone
end

smartphone = Smartphone.new
iphone = Iphone.new

smartphone.ring
#=> boop beep boop
iphone.ring
#=> boop beep boop

Everything works as expect. Now, I’ll use undef_method.

class Smartphone
  undef_method(:ring)
end

smartphone.ring
#=> NoMethodError: undefined method `ring' for #<Smartphone:0x007fd0a20b7960>
iphone.ring
#=> NoMethodError: undefined method `ring' for #<Iphone:0x007fd0a20b7938>

Not only have instances of Smartphone been prevented from responding to ring, but any subclasses of Smartphone that call ring will get tripped up when traversing the inheritance hierarchy in search of a definition of ring.