[riphay]

Long is short-hand for anyone benefiting from the price of the stock rising. This includes people who own the stock outright, have bought on credit, own call options, have sold put options, etc.

[bduerst]

Yeah, long is slang for long term investor: https://www.investopedia.com/terms/l/long.asp

[zodiac]

The link you posted doesn't support that etymology; there are some proposed etymologies here https://english.stackexchange.com/questions/145376/what-is-t...

[whatok]

That's definitely not standard industry convention. Previous poster's interpretation is correct.