Where did the usage of "Uncle" to refer to unrelated individuals come from?

We often use "Uncle" to refer to a paternalistic figure or close family friend who isn't actually related by blood or marriage. For example, I'm a godfather to the young children of a family friend, and I'm usually introduced as "Uncle John" to the kids at birthdays or holiday gatherings.

It struck me that this is rather arbitrary.

• Why isn't it, say, "Brother John" or "Grandpa John" or "Cousin John" or any other similar term?

• Does this have any origins in "Uncle Sam", the national personification commonly used in America? That's the only connection I could think of.

• Is this sort of thing common in other languages/cultures, too?

Solution 1:

'Uncle' is most appropriate since you are a male of the same generation as the kids parents.

Using family terms for non-family members is not done so regularly in English as other languages but 'Grandfather' is occasionally used to address any old man. 'Sister' and 'Bro(ther)' can be used for similarly aged people. 'Son' can be used to address any boy.

In Korean, where using someone's name directly is considered rude, the terms for "younger/older brother/sister" etc. are routinely applied to people outside the family.

Solution 2:

In Latin, the word is "avunculus", the diminutive form of "avus" (grandfather). Hence an uncle is a "little grandfather."

I suppose that calling a paternalistic figure or family friend "uncle" is a way to say that the person exhibits characteristics that are recognized as "grandfatherly."