Q From Charles F Weishar: I attempted to find the source of shirttail relative and similar expressions in Hendrickson’s encyclopedia and your site but have found nothing. I hear the phrase used to describe a person who is close but not actually related by blood.
A That’s roughly the meaning given in the dictionaries. It’s usually said to refer to somebody who is a relative by marriage or is only distantly related, such as a fourth cousin, or is a family friend with honorary status as a relative. It’s fairly common in the USA and has been since the 1950s or thereabouts.
Getting to the bottom of it, so to speak, may be a task beyond my abilities from this side of the Atlantic Ocean. One dictionary of American slang suggests it was originally southern and mid-western US dialect. The Dictionary of American Regional English (DARE) has examples from 1927 onwards, such as shirt-tail kin and shirt tail cousin, as well as your form.
Several of these sound dismissive, with a suggestion of poverty and rural, even backwoods, character. Early DARE examples suggest that they were indeed often derogatory. One from 1945 says, “Sometimes with the implication that these are not the relatives of which one is proudest”. Shirt-tail here seems in particular to be linked with poverty. There are examples much earlier of shirt-tail boy, for a young person. A 1922 book about the Appalachians remarks, “It still is common in many districts of the mountain country for small boys to go about through the summer in a single abbreviated garment and that they are called ‘shirt-tail boys’.”