The Collaborative International Dictionary of English v.0.48:

Monthly \Month"ly\, a.
   1. Continued a month, or a performed in a month; as, the
      monthly revolution of the moon.
      [1913 Webster]

   2. Done, happening, payable, published, etc., once a month,
      or every month; as, a monthly visit; monthly charges; a
      monthly installment; a monthly magazine.
      [1913 Webster]

   Monthly nurse, a nurse who serves for a month or some short
      time, esp. one which attends women after childbirth.
      [1913 Webster]