Definition from Wiktionary, the free dictionary
Possibly from the Dutch word for Boy: jongen pronounced the same.
youngin (plural youngins)
- A young one: a youth, a child.
- 1996, Mary Lee Settle, The Killing Ground 
- I swear to God, Hannah, Haley had entered that youngin in an open jumper class for children.
- 1998, Edwin Vickers, When the Magnolias Die & One Precious Summer 
- He is my youngin, yes he is; I raised him, I did.
- 1999, Silver RavenWolf, Halloween 
- He wasn’t an unlikable fellow, no he wasn’t, but he was the sort of fella that would run from honest work faster than a youngin’ can run from bath water.
- 2000, Gene S Ladnier, Fame's Eternal Camping Ground 
- If I was a youngin looking for my sweetheart, I reckon I’d hook up with Barteau’s bunch and try to cut them Yankee wagons off.
- 1996, Mary Lee Settle, The Killing Ground