It's not a word. Why are you looking it up? It doesn't exist in the English language.
"Jzilbek" is a term found in Scrubs Season 4, Episode 1: "My Old Friend's New Friend." J.D. is playing Scrabble with Mrs. Grodberg, a patient who had half of her brain removed in surgery, when Mrs. Grodberg attempts to play the word "jzilbek." Despite "jzilbek" not being a word, Mrs. Grodberg still manages to maintain a lead over J.D. in the game, even after having half of her brain removed in surgery.
"Jzilbek" can also be seen at the beginning of Scrubs Season 5, Episode 13: "My Five Stages." As the camera is panning upward, a jumble of different board and card games can be seen, and among these board games is a Scrabble board, featuring "jzilbek" spelled out in Scrabble tiles. This instance is an allusion to the earlier episode from Season 4.
J.D.: I'm sorry, Mrs. Grodberg, Jzilbek is not a word.
Mrs. Grodberg: I'm still beating you.
J.D.: Well, I'm just glad your surgery went okay, and you still have your A-Game, I don't really care who wins...
J.D.'s Narration: Half a brain, Dammit!