Furi Kuri is a favorite for some but a lot of the images didn't make a lot of sense to me. I like TheCynicClinic's video essay on the series though.
Kuuchuu Buranko is also really weird. The narrative is easier to follow but it uses a lot of weird animation techniques. You can probably describe it to be visually psychedelic. It introduced me to Denki Groove though.
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.