It comes from the Japanese language ambiguity between r’s and l’s. The title is written out in katakana as フリクリ, typically converted to romaji as “fu ri ku ri”. However, in spoken Japanese it would sound very much like Fooly Cooly because r’s are pronounced very similarly to l’s, this then becomes FLCL. If you look at the AniList page, all those versions are listed as synonym names.
It comes from the Japanese language ambiguity between r’s and l’s. The title is written out in katakana as フリクリ, typically converted to romaji as “fu ri ku ri”. However, in spoken Japanese it would sound very much like Fooly Cooly because r’s are pronounced very similarly to l’s, this then becomes FLCL. If you look at the AniList page, all those versions are listed as synonym names.