module private

test.rb

module Mod
  private
  def mod_a
    p "mod_a!"
  end
end

class Foo
  include Mod
  def public_a
    p "public_a!"
  end
  private
  def private_a
    p "private_a!"
  end
end

foo = Foo.new
foo.public_a
foo.mod_a #=> test.rb:21:in `<main>': private method `mod_a' called for #<Foo:0x007fac5b9ddbe0> (NoMethodError)