Although it became best known as a Christmas song, what is the song "Jingle Bells" originally about?

Originally known as 'The One Horse Open Sleigh' this song has an interesting history. According to Wikipedia

It has been claimed that it was originally written to be sung by a Sunday school choir for Thanksgiving, or as a drinking song.[3]

The song was one of a popular style at the time known as 'sleighing songs'. It has been suggested that

"Everything about the song is churned out and copied from other people and lines from other songs-there's nothing original about it."[5][12]

It is also interesting to note that 'sleighing songs' were popular at the time because riding in an open sleigh

gave an unescorted couple a rare chance to be together, unchaperoned, in distant woods or fields, with all the opportunities that afforded.[12]

https://en.wikipedia.org/wiki/Jingle_Bells

https://www.abc.net.au/education/what-is-music-the-surprising-history-of-jingle-bells/13647954
The second reference reinforces the 'drinking song' origins.

"Jingle Bells" was originally about an adventure during a sleigh ride.

1. A couple goes for a sleigh ride
2. The man falls out of the sleigh
3. A passerby laughs at the man and quickly sleighs away
4. The man recommends, if a passerby laughs at you after falling out of your sleigh, to race the passerby

"And crack! you'll take the lead."

The song was probably intended to be a drinking song but because of its cheerful melody, wintry theme and the "safety bells" on a trotting horse have the same rhythm, the song was reimagined, shortened repurposed for the holidays.

https://www.songfacts.com/facts/bing-crosby/jingle-bells

As far as originality, there is nothing original in topics of many songs: love, desire, breakup, sleighing, Christmas, etc. I'm sure a few songs mentioned "chestnuts...Jack Frost...yuletide carols...and folks dressed up like Eskimos" before Mel Tormé's time.