The En Mishpat Ner Mitzva on the Bavli (in the standard edition, anyway) refers to the Mishne Tora (Yad Hachazaka) as "מיי׳". What is this an abbreviation of? (I have an educated guess ("מיימוני"), but seek a strong argument or good evidence.)
It stand for מיימונידס or מיימוני - Maimonides, i.e. the Rambam. I think they used this because it is the shortest way of referencing the Mishneh Torah.