also: そい
noun
Refers to a person's action, often with a nuance of responsibility or blame. Commonly used in the pattern ～は（…の）所為だ to attribute a result to someone's doing. Compare 仕業 (しわざ), which is more common for 'deed' in modern Japanese.
See also: 仕業
これは彼の所為だ。
This is his doing.
失敗は自分の所為だと認めた。
He admitted that the failure was his own doing.
noun
Only with reading しょい
Used to indicate the cause or reason for a situation, often with a negative connotation. Restricted to the reading しょい. In modern Japanese, せい (written 所為 or せい) is far more common for this meaning.
病の所為で仕事を休んだ。
I took time off work because of illness.
天候の所為で試合が延期になった。
The match was postponed due to the weather.
仕業 is the common word for 'deed' or 'act', often implying a deliberate action, while 所為 is rarer and more literary, often used in attributing responsibility.
せい is the standard modern word for 'cause' or 'fault', used in everyday speech. しょい is an archaic or literary variant of the same kanji spelling.
The kanji 所 (place) and 為 (do, cause) combine to suggest 'the place where something is done' or 'the doing of something'. The reading しょい is a Sino-Japanese on'yomi compound, while せい is a later shift from しょい. The exact historical derivation is uncertain.