It is, obviously, a diminutive of "Uncle", and is in itself an homage to Unca Carl,* who invariably had Huey, Dewey and Louie address their elders as "Unca Donald" and "Unca Scrooge".
Note that Your Obedient Serpent doesn't have to agree with everything someone has said or done for them to receive the honorific. Sometimes, he just shakes his head sadly and says, "Yeah, there goes Unca Bob** again." Often, such lapses are instances when the person in question doesn't consistently apply his own avowed principles.
*Not to be confused with Unca Carl.
**Not to be confused with Unca Bob.
