When you get drunk and do productive work around your house. Because of your inebriation you forget all about doing the work in the morning, so it's like an Elf came and did it for you
"How was your weekend?"
"It was good. Did some elfing on Saturday night, so when I got up the spare bedroom was all painted."
effin comes from the name of an Irish Saint - St. Eimhin, pronounced Effin. So it is actually the name of a parish in Co Limerick, Ireland and some of us are very proud of it!!!