A word used by british policemen, used to explain an astounding and surprising fact in thier case. Alas the fact is quite rarely astounding or surprising.
Egads! Johnson put away your toad! You suprised and astonished me!
It is a pagan way of saying omg. It comes from the saying "ye gods", and, as everyone knows, the greek and roman gods had a way of messing with people's lives. So when you say egads, in a sense, you're saying "you fuckers," but softly.
It is used to express mild dismay or frustration.