I always assumed it was "f*k you". But after a little digging, it turns out it's not "fight yard" either. According to this comment (https://www.rockpapershotgun.com/community/profile/dmgkyd), the map was created by a 15 year old boy, who called himself Fantasy

> Why the map was prefixed with "fy_"? Because he didn't know how to put custom texture into maps, nor leave his signature. The only thing he could do was renaming "cs_iceworld" to "fy_iceworld". "fy" stood for "FantasY". He made a lot of maps prefixed with "fy_".