400 is closer, actually, but for miff, the noun it comes from (OED 1623). Where did you find 'miffy' that early? OED has 1810 for that. But the apparently irregular derivation 'mifty' which preceded it but is now obsolete, was around from 1699. Is that what you are referring to?
Yes I remember MIF. I have had a good look for it on dictionary sites, and was astonied I could only find it on one:
http://acronyms.thefreedictionary.com/Milk+in+First
Very few google hits in that sense, too, one of them oddly enough "In reference to tea, what does "miffy or tiffy?" mean?" I will answer "Milk in first (Non-U) or tea in first (U)".
I have been aged, and surrounded by ageds, all my life, but I would have thought it was more common (lol) than that. I know people who cannot contain their horror at the idea of putting the milk in first!