Jonesy is a Common Fortnite Outfit.
Jonesy is the unofficial name of this default skin you can potentially use when not selecting a skin in Fortnite: Battle Royale. I say unofficial because that name is used in the Save the World mode, but is never really referenced in the battle royale. Default Skins are generally referred to as "no skins" due to the fact that you aren't wearing a skin. This is usually in a negative connotation and describes players that are new to the game because they don't have a skin. However, there's also now "fake no skins" that are really good players at the game and purposely don't wear a skin to make other players underestimate them.
Jonesy is a bit more unique than the other ones in that it has been used with a ton of skins, including ones with masks and as mascots. This is also the one that kind of looks like TFue which people reference from time to time.
Jonesy was updated at the start of Chapter 2: Season 1.
When Epic creates certain skins they use some of these default skin templates and change their clothes and other features. I've included some of them below, but it's not meant to be an exhaustive list.