1. A lively or vivacious quality. Synonyms: vigor, energy, vitality, enthusiasm, etc.
  2. Physical attractiveness or sex appeal. Also see: it (as in "it" girl).

As far as I can tell the first one is named after the sound associated with exertion, the second with the sound a person makes when getting his wind knocked out by a strong (in this case metaphorical) blow. It's onomatopoeic, you know?