“Elf” is “11” in German. “11ty” is a cheeky way to write something like “eleven-like” or “in the manner of 11,” and “-in” can be used in a similar way (i.e. the world “Elfin” does actually mean “elf-like”). Also, if you say “Elfin” out loud it kind of sounds like “eleven”…if you do the auditory equivalent of squinting.