We don't have synonyms for shirt-tail yet — it may be a rare, specialized, newly coined, or misspelled word. The closest thesaurus entries we have are shirttail, shirt tail and sharp-tail.
(n) :
Alternative form of shirttail. [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.]
(n) :
Alternative form of shirttail. [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.]