adverb
indiscriminately; recklessly; taking it out on anyone
Describes lashing out or venting anger on whoever is nearby, without regard for who is at fault. Often used in the phrase 八つ当たりをする.
彼は機嫌が悪いと、周りの人に八つ当たりにする。
When he's in a bad mood, he takes it out on the people around him.
八つ当たりに怒鳴るのはやめてください。
Please stop yelling at people indiscriminately.
The noun form meaning 'outlet for anger' or 'taking it out on someone'. 八つ当たりに is the adverbial form.
From 八つ (many, various) + 当たり (hitting, striking), literally 'hitting in all directions'. The exact historical derivation is uncertain, but it conveys the idea of striking out indiscriminately.