Definition of God-a-mercy in US English:

God-a-mercy

exclamation

archaic
  • Used as an exclamation of applause or thanks, or (in later use) to express surprise, distress, etc.

noun

archaic
  • An expression of thanks, a ‘thank-you’.

Origin

Late 15th century; earliest use found in Sir Eglamour. From god + the present subjunctive of have + mercy, used in the sense ‘God reward you’, and hence as an expression of thanks.

Pronunciation

God-a-mercy

/ˌɡɒdəˈməːsi/