Avuncular comes from the Latin noun avunculus, which means "maternal uncle", but since at least the 19th century English speakers have used avuncular to describe uncles from either side of the family, or people who are uncle-like in character or behavior. Avunculus is also an ancestor of the word uncle itself.  Just in case you're interested, the Latin word for "paternal uncle" is patruus. 

According to the Oxford English Dictionary (OED), the first known use of "avuncular" in English was in 1831.  It appeared in the writings of Walter Savage Landor, and it originally referred specifically to the relationship with a mother's brother.

Examples of "avuncular" used in a sentence: 
"He's very avuncular, very comforting." 
"You can see this rumpled, avuncular figure making a big media hit." 
"The avuncular figure 's voice conveyed a sinister side." 
"He became a much-loved avuncular figure dispensing his own brand of wisdom." 
"A person who is avuncular is generally a kindly, supportivem and helpful older person, like a boss or teacher."
"He wasn't my uncle by birth, but his avuncular behavior of offering friendly advice and encouragement caused my entire family to refer to him as my uncle." 

41 minutes ago, drpaladin said:

Avuncular sounds like it should be some type of tram.

Or maybe, Finiculi, avuncular

Just the facts:

AVUNCULAR Definition & Meaning - Merriam-Webster

AVUNCULAR in Thesaurus: All Synonyms & Antonyms