the definition itself was actually created by a lost and now famous video of one guy and two girls putting a funnel in one girl'sass and pouring milk into it. the other girl then uses a straw and drinks the milk out of the other girl's ass. this video was around during the mid to late 90s and has since been lost. a lucky few may still have this video in their archive.
Ulzzang true definition is "best & "face" = best face in korean. They are basically people with above average looks, sometimes with the use of photoshop and haduri who post their pics online, some becoming internet celebrities.
"Hey, see that chick over there? She's pretty enough to be an ulzzang. :D"