also: せえ
noun
fault; blame; because of; due to
Used in the pattern ...のせい (or ...せいで) to indicate that something is the cause of a negative result, often implying blame or responsibility. Can also be used more neutrally for cause-and-effect, but typically carries a negative nuance.
雨のせいで試合が中止になった。
The game was canceled because of the rain.
彼
His failure is my fault.
寝坊したのは電車の遅れのせいだ。
I overslept because the train was late.
The origin is uncertain, but it may be related to the verb する (to do) or an old nominal form. The kanji 所為 is an ateji assignment.