1. (subj: human or animal) to be silent, not emit a sound
X - ни мур-мур — there hasn't been a peep (a squeak) out of X
X hasn't made a sound.
Что-то у детей подозрительно тихо. Закрылись у себя в комнате час назад - и ни мур-мур. The kids are suspiciously quiet. They locked themselves in their room an hour ago, and there hasn't been a peep out of them since. 2.-е чем (subj: human to understand, know absolutely nothing (about sth. ): X в Y-e ни мур-мур - X doesn't know a damn(ed) thing about Y
X doesn't know beans (the first thing) about Y
X doesn't have the faintest (foggiest) idea about Y.
Перегорели пробки, а сам я их починить не могу, я в этом деле ни мур-мур. The fuses blew, but I can't fix them. I don't know beans about that sort of thing.
Большой русско-английский фразеологический словарь. — М.: ACT-ПРЕСС КНИГА. С.И. Лубенская. 2004.