It's the first time I've seen those terms. I've done some research but I still can't figure it out.
Not at all sure if this is right, but a long time ago somebody told me that this would be honorary aunts and uncles, such as friends of the parents which years ago, before it was common to call everybody by their first name, were close enough not to be called by Miss/Mrs/Mr last name but aunt or uncle first or last name.
This is my understanding of the phrase.
Your shirttail might not be the best part of your shirt, being in the back. It's not what most people would like to brag about. A shirttail relative is someone who might or might not be an actual relative, but might not be the person you would speak about with glowing words.
You got me searching. I have about 15 books on word and phrase origins (including American slang). I could not locate anything.
What I related before was what my mother told me (as they say in the American south - "bless her heart").
Below is a link to something that looks good in that it refers to other sources.
Here is a link that might be useful: Shirttail relatives.
Thanks for the link, Jim.
This all started when I saw a listing for a house for sale a few blocks from our house and it said it was in a "pocket neighborhood". I had never heard of that, so I did a search and on the page linked below, I found the term, shirttail aunties and uncles.
I'm still not sure when the term came from. But it is interesting learning something new
Here is a link that might be useful: pocket neighborhood
My initial reaction is that this would not be good for me. I gotta have "my" space for doing "my" thing, Maybe for a young person or someone much older than I am, it might be good. But, I still have my flowers (that I move a lot) and my veggies (and that gets bigger every year).
Interesting, but not for me. Shirttail relatives makes sense in this situation. I have never lived in a situation that might even come close to this.