Given that it's a .jp subdomain, it's likely geared towards Japanese. Denisu is a valid shortening of DNS - the characters to spell it in katakana would be デニス, and potentially easier to remember for Japanese than "whatismyip," a fully qualified english query. Unless someone out there who knows more about Japanese than I do (admittedly, very little) can confirm that デニス or でにす is an actual word (google translate detected でにす as a phrase, giving me "su to the," which makes sense grammatically..)

Funny enough, it's also the romanjification of "dennis" (one of the first hits on google for it is dennis rodman's japanese wikipedia article).