Originally from a polite gamer sign-off ("enjoy the game"), it got shortened over time and repurposed to mean "forget it" or "get lost" — often used to shut down an absurd item-trade offer or dismiss someone outright.
A suffix shortened from "-grade student" — used as "yu-ding" (kindergartner), "cho-ding" (elementary schooler), "jung-ding" (middle schooler), "go-ding" (high schooler), "dae-ding" (college student), "jik-ding" (office worker). As youth crime has risen, harsher, more contemptuous versions aimed at middle/high schoolers have become more common than the plain forms.
UBD is a unit of counting audience numbers created using the English initials of the movie *The Bicycle King Um Bok-dong*, which failed at the box office by only attracting 170,000 viewers, and 1 UBD is 170,000 people.
"Guard up" — Korean equivalent of "clench your teeth" or "brace yourself," meaning "get ready to be hit" or "bear the pain."
"Blood rushing backward/upward," shortened — a surge of anger or adrenaline.
"Baseless confidence," shortened — confidence with nothing behind it.
It is used to mean the best of the best by adding the Chinese character 'Wang' (王) to the English word 'King' and adding 'Jjang' (meaning best). It was discovered during the 'Finding Idiot IDs' event on the DC Inside World of Warcraft (WOW) Gallery and was soon selected by the gallery members as the 'most powerful ID'.