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1.
Short for "No Exaggeration" this phrase was derived in upstate New York by an extremely lazy family who could not bother to actually say the extra syllables. The phrase was then passed through the four children in the family to their separate schools, and eventually overtook most of New York, Connecticut, and Massachusetts.
"The fish I caught was 6 feet long. No zaj, people!"

"No zaj, his penis must have been 11 inches at least!"
by Batman4realzies July 20, 2009
 
2.
Short for "No Exaggeration." Instead of saying "No Joke" which doesn't make sense.
Brian: I played Guitar Hero for like 4 hours, no zaj.
Sean: Wow 4 hours... you're retarded.
by Brian-Dizzle September 25, 2008