An exclamation coined by the Saturday Night Live skit Wayne's World and popularized by the movie of the same name.
One would generally exclaim "Schwing!!!" after the mention of a particularly alluring female. Saying "Schwing!" is usually accompanied by a pelvic thrust.
According to "Wayne's World: Extreme Close-up", schwing means (in layman's terms) "My word she is attractive".
Schwing is actually an onomatopoeia for the sound of a sword leaving its scabbard.
Think about it, you'll get it.
Mike Myers and Robin Ruzan Wayne's World: Extreme Close-up New York: Cader Books, 1991