Oof

It originated in Roblox, an online game played by nine year olds. It`s the the death sound when you fall down or get rekt by another Robloxplayer.

It is now mostly used on the internet when you don`t know what to reply to something remotely bad.

"Dude, I think I`m dying!"
"Oof."

"My dog died..."
"Oof."