The use of state abbreviations as shorthand or slang for possessive pronouns is a relatively recent in say the last 10 years, and certainly 5 and informal linguistic development. The transformation of “MI” (the postal abbreviation for Michigan) into a colloquial representation of “my” likely occurred as part of the broader trend of internet and text messaging slang.

The internet and texting culture often involves the use of abbreviations and acronyms to convey information more quickly and concisely. This includes using state abbreviations as substitutes for possessive pronouns, such as “MI” for “my.”

Having said that, this MI BABY vanity plate could simply imply that this Honda Accord is the drivers baby, or that the driver themself is in fact a Michigan Baby, i.e a native to the state of Michigan. Obviously by strict definition no one driving a vehicle is a literal baby, but could be rather young, or young at heart.