When you say "plain old" adjectivally, is a comma required between them?

Question

Examples:

• I can't believe he wore a plain old t-shirt to prom.
• I can't believe he wore a plain, old t-shirt to prom.

I could only find one dictionary that has it as a phrase:

https://www.macmillandictionary.com/us/dictionary/american/plain-old

That would seem to indicate "plain" and "old" are noncoordinate adjectives and so don't require a comma between them. I've never heard of that dictionary, though, and I didn't find entries for it in the two universally-accepted-as-establishative dictionaries, i.e., Merriam-Webster's and OED. That leads me to believe they may be coordinate adjectives, so now I don't know.

Also, does it make a difference if by "plain old" I mean the t-shirt is an ordinary, nothing-special-about-it t-shirt or if I mean the t-shirt has no writing on it and isn't new? Are those even different meanings? I'm not sure. It seems like maybe they actually mean the same thing and I'm splitting hairs.

Or maybe I'm completely off the reservation and they're neither, not coordinate or noncoordinate adjectives, meaning they're not two serial adjectives but one single adjective, a phrasal adjective? If so, if "plain old" is that instead, then does that mean that rather than a comma I should put a hyphen between them?

Example:

• I can't believe he wore a plain-old t-shirt to prom.

Which of the three examples I've given is the right way to write it? And if there's more than one right way to write it, does writing it different ways change the meaning at all?

Answer 1

• He wore a plain, old T-shirt = the T-shirt had no pattern or image on it.

• He word a plain old T-shirt= plain is colloquial for the word ordinary, and plain old is also a cliché in American English. This meaning is in the Merriam Webster.

With a comma it means meaning lacking special distinction and without a comma plain is the same as saying: regular old T-shirt. In this sense, it's a quasi-adverb modifying old. Oxford Dictionary: A smallish group of adjectives are idiomatically used in such a manner as almost to fall into the broad class ...

This meaning is in the MacMillan Dictionary:

not at all unusual, interesting, or special The recipe is nothing special – you just use plain old hamburger and some onion powder. plain old common sense

Synonyms and related words Not interesting or exciting:boring, dull, tedious...

plain, lacking ornament or undecorated versus lacking special distinction or affectation