1. quite young and immature in behavior.
  2. Chiefly Midland and Southern U.S. of distant relation, especially by marriage: some shirttail cousins I'd never met.
verb (used with object)
  1. Informal. to append or add (an item) to a discussion or writing: The tariff concessions were shirttailed onto the trade bill.

Origin of shirttail

An Americanism dating back to 1835–45; shirt + tail1 Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018

Examples from the Web for shirttail

Historical Examples of shirttail