The full form of MMY is Many More Years
MMY stands for “Many More Years”. It’s a good-wishes acronym that can be used at many different occasions including birthdays, holidays, weddings, etc. It can also be used in a sarcastic manner, generally when things aren’t going well and someone just accepts that these things happen or will continue to happen.

Origin of MMY
The likely origin of this acronym was with emails in the early-to-mid 1990s. Sending someone a email regardless of the occasion was one of the fastest ways to get a celebratory greeting across that didn’t involve a phone call. This slang word saw solid use throughout the rest of the 1990s and into the 2000s, even making the jump to text messaging. By the end of the 2000s its popularity had waned and near the end of the 2010s MMY has fallen into obscurity.

Conversation Examples
Text Between Friends

Friend 1: “How’d your final go?”
Friend 2: “Guess.”
Friend 1: “Knowing you I know you passed.”
Friend 2: “With a 99% Perfect As all through highschool!”
Friend 1: “Awesome! I knew you could do it. You’re really smart! MMY to your smartness!”
Friend 2: “LOL! Thanks!”