From what I could tell, Raku appears to be from Rakudo, the Perl 6 compiler, which is a shortened form of rakuda-dou (="way of the camel" in Japanese). Rakudo also means "paradise". The "raku" from "way of the camel" means "camel", while the "raku" from "paradise" means "fun" or "enjoyable" (or "music").
Incidentally, it also happens to sound similar to "roku" (="six").
Incidentally, it also happens to sound similar to "roku" (="six").