Green’s Dictionary of Slang

agad! excl.

[gad n.1 ]

a euph. excl. meaning God!

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1751
[UK]Villiers Rehearsal I i: A gad, I’m sure they are not able to do themselves.
[UK]S. Centlivre Gotham Election I i: Five Thousand Pound! a-gad.
[UK]Smollett Peregrine Pickle (1964) 323: But, agad! you was never more mistaken in your life.