Honest to God/goodness
Definition 1 of 2
Pro Tip 1/3Emotional Stress
Shows strong feeling or urgency about being truthful. 
I missed the meeting, honest to God, because my car broke down!
Pro Tip 2/3Casual Conversations
Common in informal talk, not suitable for formal situations. 
Honest to God, that was the best burger I've ever had.
Pro Tip 3/3Declaration of Truth
Used when you want to be believed, insisting on your honesty. 
She left early, honest to God, I'm not covering for her.