Can be used in two different situations. One, as a substitute for no kidding when responding to an obvious statement. (eg. Today is Saturday? Der!) Two, to express exasperation over a situation. (eg. You mean I have to get up at 9:00? Der!) Example: Statement: You know, if you add two and two, you get four. Response: Der!
DER
Meaning of DER in English
Slang English vocab. Английский сленговый словарь. 2012