Abbreviation of 'shit yourself', often used as an alternative to 'kys'
Person 1: 'Hhhaha! Domb Tomb is such a fun and cool game!!'
Person 2: 'That's cringe'
Person 1: 'sys'
SYS = Sucking YourSelf
Some men have the amazing ability to suck his own cock! Also, some women have the skill to lick/suck her own vagina...
Wife: "I'm sick of you... Go suck yourself!"
Husband: "Gladly! I do it better than you anyway... I have my own SYStem."
Girl 1: "I won't lick your pussy..."
Girl 2: "God... I'll do it myself! Thank god for SYS..."
Boy 1: "Wow, dude.... you can do auto-fellatio? That's amazing!"
Boy 2: "......." Looks at Boy 1, smiles and thinks: "SYS rocks..."