Party

The word “party”  arrived around the thirteenth century from an old French word that meant “that which is divided”. Think “partition” or “party wall”.

The political sense of “side in a contest or dispute” evolved by 1300.

Later, a party as a “gathering for social pleasure” was first used in 1716, originally for some specific purpose,
e.g. dinner party, hunting party.

So we went from a group of politicians, divided from their opposition, to a group just having fun and getting pissed.

It’s a shame we didn’t come up with a new word for the latter.