One had a shirttail hanging outside her trousers and a toothpick in her mouth.
From the post-gazette.com
He is the anti-country clubber with untucked shirttail and dangling cigarette.
From the usatoday.com
He was wearing blue tennis shoes and white shorts, his shirttail hanging out.
From the time.com
When Isaac, 5, walks into the chapel with his green shirttail out, Karla doesn't flinch.
From the sltrib.com
Roll up the sleeves, let your shirttail roam free and it can sub for a tee on most occasions.
From the washingtonpost.com
When I was a shirttail kid living with my folks in the country, we did not produce garbage.
From the chron.com
Nobody was harmed, and the goat appeared to have fully recovered as he chewed a reporter's shirttail.
From the online.wsj.com
Guys can be fat, slovenly, one shirttail in, one out.
From the sacbee.com
It always keeps its tie straight and its shirttail tucked in.
From the sacbee.com
More examples
A brief addendum at the end of a newspaper article
Fabric forming the tail of a shirt
The single or split (then rather plural) bottom part of a shirt, below the waist, especially in the back, which, when not tucked into trousers or other vestment, hangs over the wearer's tail-end, like a tail
A scoop in the front and/or back of a shirt that is longer than the sides.