～みだりに

This expression carries the meaning of "without necessity", "without permission", or "on one's own accord".
✦It often follows statements of prohibition, such as 「みだりに...するな／してはいけない」(don't do something recklessly). In everyday spoken language, it's often expressed as 「勝手に...しないでくだし」(please don't do something without permission).
✦This is a formal and written style of expression.