self-serving logic; convenient reasoning
Refers to reasoning that is twisted to suit one's own interests or convenience, often used critically.
His argument is self-serving and completely unconvincing.
自分勝手 describes selfish behavior in general, while 手前定規 specifically refers to self-serving logic or reasoning.
ご都合主義 is a broader term for opportunism or expediency, whereas 手前定規 focuses on the act of twisting logic to fit one's own convenience.
Compound of 手前 (oneself) and 定規 (ruler, standard), literally meaning 'one's own ruler,' implying a standard that is conveniently adjusted to suit oneself.