test.rb

class Human
  def initialize(name)
    @name = name
  end
  def define_method(name, &block)
    self.class.class_eval do
      define_method(name, block)
    end
  end
end

taro = Human.new("Taro")

taro.define_method(:who){
  puts "I'm #{@name}."
}

taro.define_method(:hi){ |to|
  puts "#{@name}: Hi, #{to}."
}

taro.who                        # => I'm Taro.
taro.hi "James"                 # => Taro: Hi, James.