Phonetic approximation errors are common when recalling Japanese by ear:
It is also possible the phrase includes another language. If “mal” is Malay for “bad” or “wrong,” the phrase might be a hybrid: “Relative’s child and an overnight stay, so that’s bad.” No known cultural source supports this. shinseki no ko to o tomari da kara mal
If the user intended something like:
“親戚の子とお泊まりだから、悪い” (shinseki no ko to otomari da kara, warui — “Because I’m staying overnight with a relative’s child, it’s bad/awkward”), here is a long article on that topic. It is also possible the phrase includes another language
Language is slippery, especially when attempting to recall phrases from foreign media, anime, or songs. The phrase “Shinseki no ko to o tomari da kara mal” presents a fascinating case of probable linguistic fragmentation. While it has no direct meaning in standard Japanese, breaking it down phonetically and semantically reveals several plausible sources. Language is slippery, especially when attempting to recall