Nobody gets permission from Webster's to make up terms for rifles. A Poor Boy Rifle generally lacks any frills, like Brass furniture. Most do not have a buttplate made of metal. Some had leather buttplates, and others none at all. No patch box, but some had a hole drilled into the stock to hold grease. Furniture was usually iron. No carving, inlays, etc.
These were working guns, for mountain folk, who just could not afford to pay for all those extras. They were made right on up into the 20th century, and represented their own distinct style after a number of years. I have never been clear when the term " Poor Boy " was attached to these guns. When I was young I heard them called all kinds of names. When I was looking for a rifle in the late 70s, a firm was offering a " Poor Boy " rifle, and shortly after that, Dixie Gun Works began marketing a similar looking rifle they had made for the company in Japan. The advertising made the name stick.
The most common design distinctions for Poor Boy rifles is the hole in the stock for grease, and then the shape of the Trigger guard, which is usually rectangular rather than oval in shape, again, made or iron or steel. But, not all original Poor Boy rifles had trigger guards of this shape.
I specifically requested that a hole NOT Be drilled in my gunstock for grease, and have since seen several originals or pictures of originals that lack the grease hole, and of course, many replicas that are lacking the grease hole.
And, you see builders using that style of trigger guard on other kinds of rifles, that have brass pipes, trigger guard, buttplate, toeplate, sideplate, etc. I have not yet see this style of trigger guard cast in brass, however. Someone probably has done it, but I haven't seen one yet.
I suspect that the POOR BOY name is something of modern invention, but I can't find any source of information on the origin of the name. The most common name I remember hearing being used 40 years ago to describe these guns, was just " Southern " Mountain rifle.
"Southern Mountain Rifle" was used to distinquish these guns from the more traditional Southern rifles, made in Virginia and the Carolinas, that had both wood and metal patch boxes, and brass or german silver furniture. Sometimes, you see German style lockplates used on the Poor Boy rifles, where most Southern rifles were made with English Style locks.
I hope that helps answer your question.