Vacantly; blankly; in a daze — a mimetic sound for being stunned or spaced out.
彼は突然の知らせにぽかんとして言葉が出なかった。
He was dumbfounded by the sudden news and couldn't speak.
子どもはお菓子を見てぽかんとしている。
The child is staring blankly at the sweets.
Openmouthed; gaping; with one's mouth wide open — used when someone is astonished or stunned.
彼女はあまりの美しさにぽかんと口を開けた。
She opened her mouth in astonishment at how beautiful it was.
その光景を見て皆がぽかんと立ち尽くした。
Everyone stood there gaping at the scene.
冗談が面白くてついぽかんとしてしまった。
The joke surprised me and I couldn't help but gape.
With a whack; with a thump — a mimetic word for a light impact or thumping sound.
子どもが風船をぽかんと叩いて割ってしまった。
The child gave the balloon a thump and popped it.
本棚から落ちた本が床にぽかんと当たった音がした。
There was a dull thump when the book that fell from the shelf hit the floor.