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I vaguely remember some phrase used in Torah discussions such as "moni palomy" or "moni plamoni" that means "John Doe" or "What's his name" or "John Q Public". Seems like it was used in Ester, but I cannot find it.
You might be thinking of P'loni Almoni פלוני אלמוני, used first in Ruth 4:1 as a placeholder name. (See here for its etymology and why it must be a placeholder name.)