Egad

[Sarah]

An exclamation expressing exultation or surprise, etc.

[Jessica]

An interjection or exclimation, usually due to surprise. Sometimes said by the eldery folks.

[Tina]

Egad is a morphed form of egads.
Egads used to mean ye gods and now is more closely linked to OMG or oh god; Some people think there is only one god, so egad has evolved to express that idea. At the heart of it is the notion that god is a trickster who screws with us.
We use it to say that something severely messed up has just happened.

[Gregory]

[oh my god], [put] into a [shorter] and more convinent way

[Justin Bieber]

Expressing surprise, anger, or affirmation.