Why was 'Endgame' so called?

About the title for 'Endgame,' there are several possibilities:
BECKET'S PLAY: Samuel Beckett's play 'Endgame' deals with a couple (both men) who stay together even though they don't get along, pretty much the situation with Victor and Margaret. The two characters are waiting for the end of their lives, which eventually will be repeated in a sort of reincarnation. The word 'endgame' is used in chess to describe a situation in the game when most of the pieces have been sacrificed, although the principal pieces (the King and the Queen) remain. Both game and play are about finality, the end of a match, as it were.
By the way, Beckett's play was written as a theater-of-the-absurd piece, which characterizes the bizarre nature of this episode of OFITG.
MARGARET'S NEAR-DEATH: Margaret suffers a heart attack and almost dies. In other words, her life almost comes to an end. (Actually, she WAS clinically dead for several seconds but recovered.)
MR. SWAINEY'S MOTHER: She comes to an end in this episode, having never been seen on screen.
THE HAUNTED CARAVAN: Victor's dream of touring Britain in a trailer comes to an end.
VICTOR'S 'GAS PROBLEM': Victor's worries cause him a nervous colon that creates havoc with his rear end. (Rear END, get it? Wink-nudge-nod.) Knowing scriptwriter David Renwick propensity for puns and plays-on-words, I wouldn't be surprised if 'Endgame' was instrumental to the creation of those scenes that showed Victor rushing to the bathroom and playing the radio loudly to block any noises from his "very gurgly gut."
ALL OF THE ABOVE: I vote for a combination of all those elements.
For the record, 'Endgame' was the Christmas special for 1997. It was first broadcast on 25 December of that year – the year's END.

I took it as a reference mainly to mortality and taking a good, long look at the ultimate realities of life what the characters are looking for (i.e. happiness, harmony together): the endgame, possibly. It's sort of about getting to the end of various things and how that can happen, not to mention what "the end" really is.

'Irregardless' is not a word

The previous special, The Wisdom of the Witch from 1995 was due to be the final episode and feature the death of Victor, dying a hero saving Patrick from the man who is trying to kill him. The scenes of Margaret sitting by Victor's grave was meant to be her Victor, not his Dad's. They filmed Victor bandaged up complaining about the television at the last moment.

Endgame was also meant to be the end of the series but this time Renwick wanted Margaret to die, having a heart attack due to the stress of living with Victor. But once again, during production, they changed their minds.

As we now know it was another few years before they decided once and for all to kill of Victor.

So Endgame was so called as it was due to be the final episode.