1. To misspell the phrase, "See you!" when writing good-bye.
2. Used to describe a petite Asian girl of unnatural strength, on par with the fictional character "Chewbacca," from Star Wars.
1) "You idiot, it's spelt 'See you!' not 'Siyu!'"
2) Guy 1: "Wow. She just totally picked up that dude and put him in the trash can. She's such a Siyu."