It was originally Reddit is Fun, but then Reddit asked a bunch of app developers to change the names of their apps to make it clearer that they aren’t official

It was originally Reddit is Fun, but then Reddit asked a bunch of app developers to change the names of their apps to make it clearer that they aren’t official or affiliated with Reddit directly, which is why most apps are called “X for Reddit”. Reddit is Fun was already usually called RIF, so it became a recursive acronym. Then I assume they added “for Reddit” for search/discover ability reasons