arbitrarily; of one's own accord; without asking; acting as one pleases (often used as '勝手に〜する').
勝手に部屋に入らないでください。
Please do not enter the room without permission.
彼は私のスマホの設定を勝手に変えた。
He changed the settings on my phone without asking.
勝手に決めないで。みんなの意見を聞こう。
Don't decide on your own. Let's hear everyone's opinion.
テレビが勝手に消えたのでリモコンの電池を替えた。
The TV turned off by itself, so I replaced the remote batteries.