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1.
As a noun it means a person that acts stupid. When used as an adjective, it means retarded. It can also be used as an interjection meaning, "yay!"
(Noun) Nick: Why is that kid touching his butt?

George: I don't know, but he is a total schna.

(Adjective) Billy: That's a schna thing to do with a puppy.

(Interjection) Julian: Schna, schna, schna! I love puppies!
by OurSavior_Jonathan December 05, 2010
 
2.
not mucus related. in the onion/garlic family.
schna is in aisle 3 next to canned nostrils
by #27907 November 14, 2007