[VPsubj; these forms only; fixed WO] some matter is so convoluted, intricate, or confused that it is impossible to understand, make sense of, or clarify it: сам чёрт ногу сломит (в Х-е) - it (X) could trip up the devil himself; the devil himself would be stumped (at a loss); s.o. would have the devilfc own time with X (trying to make sense of X).
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♦ Анкеты бывают разные. Бывают попроще, бывают потруднее, а бывают такие, что черт ногу сломит (Войнович 1). There are various types of questionnaires. Some are a bit easier than others, some a bit more difficult; then there are those that could trip up the devil himself (1a).
Большой русско-английский фразеологический словарь. — М.: ACT-ПРЕСС КНИГА. С.И. Лубенская. 2004.