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"
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.