The word to use when you don't know what to use. This word can be used in many cases such as a scary moment, when you forget or don't know what to say, or even (my personal favorite) when the room is extremely silent and you want to make some friends.

something that you scream when you are being mugged by a really scary dude with a pistol

The answer to all life's happenings.

a sound you make when you moan

means scream or pain

the thing you say when you are at work and there is so much work ahhhhhh