Back during the D1 and D2 days, a couple of friends and I used to have LAN parties, like many here.
We would sit up and play D1 and D2 for hours on end. During one of our play throughs of D2, we were rescuing Deckard Cain and noticed that we saved his a$$ from the cage and he took a town portal up and left us to fend for ourselves agains the monsters.
That s.o.b. closed his portal. I shouted "We have all just been Deckard Cain'd"
That was many years ago but ever since then we have used the "Deckard Cain'd" term for many things. Even in D3 for instance, I would Deckard Cain my friends by going down in a massive pack of monsters, pulling as many elites and champions together as I could while my friends were up in town. Then I would tell them they can just come down my banner its safe.
They are greeted with an insta-death sometimes and I get to smack them with the "you have been deckard cain'd"
one more example, is go drag a bunch of fallen maniacs back to your friends and let them eat the explosion in the face for a quick "deckard cain"