Could you say anything about where you heard it? A movie, maybe, or a video game? AE or BE? Any context would help a lot. Without it it's like searching for a needle in a haystack.
Was the accent 'ménu' or 'menú'?
Was it like English /'mɛn ju:/, or more like German or French?
This is probably going out too far on a limb, but is there any chance the context had to do with Latino or Chicano culture? If so, could it be 'menudo'? (= mexikanische Kaldaunen-/Kuttelnsuppe) https://en.wikipedia.org/wiki/Menudo_(soup)
You will be in deep menudo, pal
You will be in a deep menudo pot
Otherwise, the expression 'in deep _______' often has some creative equivalent of 'sh*t,' for which there are many synonyms. Doodoo, caca, guano ...