Definition from Wiktionary, the free dictionary
ellipsis of a full oath of the form "I swear by God that…"
- Used as an oath to emphasis the veracity of an associated statement.
2007, Cassandra King, Queen of broken hearts:
- With a frown, Rye studies my face. He disengages my arm in order to take my hand in both of his and squeeze it tight. "Why don't you go back and confront him, sweetheart? I'll go with you, by God. I don't like the idea of him bullying you, and he needs to hear that."
- Used as an intensifier.
1991 November 20, Paul Hendrickson, “Echoes Of the Day of Infamy”, in Washington Post:
- I saw that map and I said right then, 'Someday I'm going to make a map of how it was that day, make a drawing of Pearl Harbor, and I'll make it right.' And I did, by God.
- Used as an exclamation of surprise.
- By God! That chicken has no head!