honest-to-god

Also, honest to goodness or Pete; honest Injun. Truly, really, as in Honest to God, I didn't know it was yours, or Honest to goodness, we had exactly the same experience, or I promise I'll finish in time, honest to Pete, or Honest Injun, I didn't take your wallet. These colloquial assertions date from about 1900, except for honest Injun, dating from the late 1800s and today considered offensive.

The adjective has one meaning:

Meaning #1: used informally especially for emphasis
  Synonyms: honest-to-goodness, old, sure-enough