A Nuncle an uncle. The word is a metanalysis (mis-division of words) stemming from the phrase "mine uncle" shifting into "my uncle". It likely came about in the late 1500s, and nowadays is almost entirely unused. The only place I've come across it was in the A Game of Thrones books, which make copious use of archaic language.
There's also a verb to nuncle, meaning to cheat or defraud. The OED lists this as British, regional (being local to Leicestershire, it appears), and rare.
Maevwyn points out that the noun also appears in The Lord of the Rings, in Sam's song/poem Troll sat alone on his seat of stone, and both raincomplex and tubular, being considerably better-read than me, point out that it's used in King Lear.