I tried to search on dictionary but seems the term "party" is used in videogames "out of context".
Party: a social gathering, as of invited guests at a private home, for conversation, refreshments, entertainment, etc.
It seems to have nothing to do with a RPG Party (maybe in a loose interpretation). Seems that someone at some point just started to use that word and people liked the word for that use. Recluting someone ( Hei join the party!) into your group to go killing a boss is not exactly the same as a social gathering at your home.. or maybe yes? at least you are going to boss' home (uninvited).
Was that a carryover from tabletop games (see comment here)?
Best Answer
The definition of "party" you have found is only the first, most used, and best known definition of the word. It's the definition that is closely tied to celebration and fun. In short, it's a party!
There are other definitions, such as a political party, which is (likely) quite the opposite of fun.
Here are two other definitions which would fit the traditional RPG party:
Obviously, taking down a boss is an activity, and it often involves a trip to the boss' lair.
In this case: the hero's party vs. the bad guys' party.