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1.
It means an extremely dumb person..beyond dumb acutally. I believe the origin is someone being at such a loss for words that it's a combination of many cuss words.
Whore #1: I'm tired, I've got cramps and my feet hurt. Can I call it a night and go home and lie down?

Pimp: You, you---you's a cubba?

OR

Elle: How was your date with Mr.Sexy

Ella: It sucked...he's such a cubba!

Elle: Oh, yeah...he's from St. Olaf