- A lively or vivacious quality. Synonyms: vigor, energy, vitality, enthusiasm, etc.
- 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?