It's the same as der(see urban dictionary), but it is to be used in a more manner-oriented setting.
It's Der+Sir=a more respectful DER
You can use it at formal parties as well as with your friends if they've had a bad day, but they've said something stupid. You have to "der" them, but you don't want to break their spirits completely so therefore a simple "dersir" will suffice.
Dave was at the charity Gala and people kept telling him stupid things so he would respond to them with an abrupt, polite "dersir"