せいでBecause of, Due to, Blame

Although written almost exclusively in hiragana, せい is a noun that may be better understood from its kanji, 所為(せい), meaning 'doings'. せい is used to convey negative results that occurred due to the behavior or actions of another person/thing. Because of this, common translations of せい are 'fault', 'due to', or 'because of'.

As せい is a noun, it may be used after the attributive form of any other word. The case marking particle で will also regularly be seen after せい, especially when conveying the (B) result that occurred 'with' or 'by means of' those 'doings'.

Fun Fact

The 'doings' that 所為(せい) represents is similar to the negative 'doings' that is used in English to place blame. This can be seen in phrases such as 'it was the weather's doings', or 'ah, so the missing cookies were your doings!'