Debygawd Cap-en! Where does this phrase come from?

Solution 1:

It appears 'calls' to God and variations of by God have geen in the English language for centuries:

pur DEE BY God in Shakespeare

and

pardie, int. (and adv.) and perdie and pardi and pardee OED

• Now archaic and rare.
• ‘By God!’ (as an asseveration). Hence: ‘certainly!’, ‘without a doubt!’, ‘indeed!’ Also occasionally as adv.

As in:

• 1387 Chaucer Canterbury Tales Wel koude he stelen corn and tollen thries, And yet he hadde a thombe of gold, pardee.

1905 A. C. Swinburne Poems III. 134 For all my subtle wiles,
perdie, God wot I loved him well enow.

• 1930 R. Kipling Miracle of St. Jubanus 7 He was of exemplary
  life. Pardi, he had to be!

The reference found by other members: Indeed, By God appears to be a variation of the same plea/citation to a deity and is specific to the OP's question.