Pow is a sound-word, not a description. In children’s comics and old cartoon fights it stands for a gunshot, an explosion, or a hard hit — often printed huge: POW! It is playful on purpose; nobody in a real news story writes pow when a window blows in. You can shout it as you mime a punch, or use it as a tiny noun for that kind of bang, though actually people mostly just drop the word in as the sound itself. Informal, comic-book English. Other cartoon hits sit beside it (wham, bam, bang); pow is the short, sharp one. Grown-ups use it with a smile, quoting the comics, not as serious language.