So also likely to be used by Norwegian Americans often found in quantity in Minnesota & suchlike.
Spotted on a Jeep: MPG LOL
On the back of a Ford Taurus: MADDAG6
The plate frame was a bog-standard dealer’s frame and there were no stickers or other aids for the meaning. The 6 on the end would make it look like part of a company fleet but there was nothing to indicate it wasn’t a plain ol’ private auto.
Maybe some other people already took MADDAG, and MADDAG0 through MADDAG5.
Tonight I saw RUBER. He damn near merged into my front fender, so I got a better look at his plate than I wanted.
The only “ruber” I know is a long departed poster pseudotriton ruber ruber which is the genus and species name of the Red salamander - Wikipedia.
So yeah, I thought that guy had the driving skill of a salamander.
Was it on a red car? “Ruber” is a Latin word for “red,” hence the name for the red salamander.
Maybe. So what’s a Maddag?
Twenty bucks, same as downtown.
Nope; car was black or dark gray. It was night and hard to tell. Perhaps the driver is a redhead or has “Red” as a nickname for some other reason.
Beats heck outta me. A “hip” way to say “mad dog”?
The Attorney General for Mothers Against Drunk Driving?
Which was kind of my point. There is no apparent meaning but it’s popular enough there are six, maybe seven, iterations out there?
I just now did some googling. I think I have our answer.
For sure this is not my area of expertise and I bet it’s not yours either. But one hell of a lot of people play World of Warcraft.
Today I saw a generic SUV with PROV⎵III.
Are they Provo the third? I wondered if they meant chapter 3 the biblical Book of Proverbs, but that’s not the typical cite form.
I think we have a winner. When I googled I got a picture of a frog and a Swedish(?) cookbook. I should have scrolled down further.
Saw a good one today in the grocery store parking lot. Just said GET 1. Which didn’t mean much at first until I saw the license plate frame that said “Mammograms Save Lives”.
But shouldn’t you get two? One for each side? 
Well, for those who need a backbone maybe SPINES has extra ones to give out?
There was nothing on the vehicle to indicate anything special like they work at doctor’s office, or chiropractor or anything of the sort.
//i\\
PS: thanks to @LSLGuy for the mormor likely definition. I do know of people who have worked on getting a customized plate for their mother/grandmother so maybe it was a gift.
Maybe the car was a gift. I gifted my Mom a car once.
Whaps @LSLGuy with a rolled-up newspaper 
Had a two-fer this morning. First was a gray SUV with STILL ME, then a white SUV with Michigan specialty plates that I think had a bluebird on them with SNOWS.
Today I saw a Kia SUV with a standard FL plate with CUTOS2U. I even took a pic to be sure I got all the letters right. No other informative markings on the car.
The only thing I can think is it means “kudos to you”. Perhaps for figuring out the plate? if so, How very meta of him.
Although it might be “cuddles to you” in cute baby-speak. The 30-something dude driving did not seem the cuddling type though.
Hmm.
This is on an Aston Martin Vantage F1 edition, spotted in the supermarket.
That’s an example of a fake license plate. That is not a genuine Massachusetts plate.