It means, "the dark sword". The word "gen" can also mean mysterious, profound. Genbu 玄武 from which the Japanese borrowed from the Chinese means "the dark warrior" representing the North direction. The animal associate with it is the tortise. East is seiryu (green dragon),est, Bakkyo (white tiger) and south Suzaku (red bird). Therefore it can be also translated the "sword of the north".

玄 = mysterious 釖 = sword You can work it out from there.