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1.
old boy In Kempston, just as people say 'old man' as a term for their father, we also use 'old boy' to refer to our elder brothers. In the same way, 'young old boy' is used to refer to our younger brothers. This appears to be contained within Kempston and is dying out.

Half-brothers and step-borthers are also sometimes described in this way.

It is mostly only used in the genetic sense but is seldom used in terms of emotional bonds.
Excerpt
Person A: Aren't you somebody's little brother?
(person's B's response here)
Kempstonian person: Yeh, they're my young old boy.
by Walking Talking Dictionary August 10, 2006