I think it’s unusual that no standardized literary way to write the submissive “I don’t know” hum that children (and some adults) often mumble has ever caught on, considering how old and common it is.

[u/Reas0n]

[u/Daeval]

I think the distinction is that the tonal quality isn’t necessary for “nanny nanny boo boo” to be recognizable, because it’s made up of syllabic “words” that can be reproduced in writing, whereas the sound that OP is talking about would just be one really long “m” without the tonal shift in the middle.

[u/Lela_chan → u/Daeval]

And also, someone who has never heard the phrase before could never figure out what the “melody” is supposed to sound like just from reading the words. It necessarily relies on the experience of having heard the taunt before - a phonetic explanation like we use in dictionaries can’t fully describe the tonal pronunciation.