The term was originally a play off of the name of the company "Nitrous Oxide Systems", whose logo is basically the capital letters N-O-S.
It's use is mainly limited to the import or sport compact side of the automotive performance scene, though in many cirlces the use of this slang is actually looked down upon (where the prefered term is nitrous ).
To act weird in public just for grins. To play a practical joke. To try to freak people out by calculatedly bizarre behavior. To mess with peoples' minds.
NOUN: The act itself.
"Don't mind me, I'm just nossing."
"Do you think his act was serious, or was it just a big noss?"
"I noss, therefore I'm weird."
Term coined in Indianapolis, Indiana in early 1970's.