What is the meaning 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.

The tail-end or periphery of something.

A tenuous connection.

A distant kinship.

A small portion

Something small and unimportant.

Source: wiktionary.org