easy to pronounce, even for children (though kids 4 & younger may pronounce it more like "mayd" or "maygh")
ONE SYLLABLE!!! ("individual" is too goddamn LONG.) you have to be able to say it quickly and casually
ends in a soft consonant sound, so it'll flow right into the next word ("ind" halts the whole sentence)
fits neatly into the existing structure as a relative of master/mistress that can be abbreviated down to an m and one other letter
distinct enough that it can't be mistaken for either gendered term (if you call me mix I'll kill you. it sounds like miss with extra steps)
yes, i know about milligrams and magnesium. i don't give a shit. ms can also mean microsoft. who cares.