This is a Japanese word meaning more or less "you're dead", an equivalent of "shah mat" (the king is dead) in chess. It is the final expression in a card game called Duel Masters (or Kai-judo), based on which several computer games have been created (basically it's kind of battle between monsters). During the game you're supposed to use several other commands in Japanese, like "ketsy da" (I'm challenging you), the adversary says "yoshi" (I accept) later on there are some commands "iku-zo" (start), "ike" (you go) ets.

"todome" on itself is a standard Japanese word to be found in any dictionary, meaning "the decisive blow"

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Note added at 1 hr (2008-04-20 09:54:48 GMT) Post-grading
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Todome Da! - "The killing blow!" by thethongofanime Oct 13, 2004 email it ... Unlike other animes and yugioh, Duel Masters lacks any proper plot and ...
www.urbandictionary.com/define.php?term=duel mast

Of course the word may be used in other games as well :)