Often accompanied by the "hit-it" hand signal.
Also has many derivitives or shortened versions used to express more than just a hot lookin' fanny. Such as: Pow, Powzer, Powpowpow, ect.
A cry out of awesomeness, satisfaction or accomplishment.
Can also nonchalantly pronounced, slightly wearily.
"I just tied my shoelaces, kapow!"
"Amsterdam is just kapow!"
Looking in the mirror. "Kapow!"