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1.
essentially used as a filler word.

the meaning of it changes depending on its context.
"dude, i just got my phone jacked! im soooo rizzing!"

"omg! i got a full ride to Harvard! this is soo rizzing!"

"you need to shut up. you're rizzing and its seriously irritating me"
by okimangiee April 19, 2010
 
2.
Extremely embarrassing to an unbearable degree; used to describe someone who is so shockingly embarrassing, you can't even stand to be in the same vicinity as them
Oh my god, he is SO rizzing.
by Paolo Rabbit November 16, 2008