- an exclamation of surprise and awe
- a thing or event which is unbelievable (can be used both in negative and positive contexts)
1. When used as exclamation
A: 어제 카페에 갔는데 지드래곤이 나오고 있지뭐야 (When I went to a cafe, I saw G-dragon getting out).
B: 와 대박! (Wow, Daebak!)
2. When used as a noun
A: 전여친한테 헤어진지 1년만에 문자 왔어. (Yesterday my ex-girlfriend sent me a text message a year after we broke up.
B: 대박이네 뭐래? (It's a daebak. what did she say?)
Dae in daebak means big, and bak refers to a kind of gourd. So, it literally means a big gourd. There's no clear evidence when a big gourd came to be used as an expression of surprise. However, Heungbu & Nolbu Brothers, a well-known Korean folktale obviously has a huge impact on the recent usage of daebak.
In the tale, a big gourd brings unexpected (and fortunate) events to the each main characters. When the main characters cleaved the big gourds in two with a saw, they get a variety of unexpected outcomes. The big gourd symbolizes a surprising, awesome outcome and event in the tale.
If you are interested in the further story, check out following link: http://www.tparents.org/Library/Unification/Talks2/Kirkbride/Kirkbride-100629.htm