A term brought by Audrey Tang into Perl 6. It means simply: optimized for fun. Or more articulate: Go after the idea you enjoy most. Pugs was allegedly developed this way and grow thatswhy in many directions. Other projects like KindaPerl6 came out of this.
-O is the command line option for compilers, telling for what goal to optimize (speed, memory, *). Of course a compiler can't optimize your once written software for the fun using it, but we human can optimize our doings for maximal enjoyment. This was always a strong part of the Perl philosophy.