Green’s Dictionary of Slang

1. (US) a general intensifier, e.g. the by-god worst thing ever; also as n.

1884		C.F. Lummis letter 25 Dec. in Byrkit Letters from the Southwest (1989) 188: To quote an eloquent section-hand, ‘this is the bigoodest country for water on the face of the earth’.
1935		S. Lewis It Can’t Happen Here 206: Insulting the whole by God corpo state.
1956		N. Algren Walk on the Wild Side 89: ‘Ain’t this the By-Goddest weather you ever seen?’ ‘I’ve seen By-Godder.’.
1971		T. Thackrey Thief 291: I had heard the bulls thereabouts were just tougher than a bygod (and stupider than a post).
1974		(con. late 1940s) E. Thompson Tattoo (1977) 477: Hotter than a by-God, ain’t it?
1987		‘Joe Bob Briggs’ Joe Bob Goes to the Drive-In 146: They never learned to shave their armpit hair over here. That’s a true by-God act. Pitiful.
2005		D. Jenkins Slim & None 204: ‘This is how my mama fixed it, and how her mama fixed it, and it’s the only by-god way to fix it’ .

2. as infix.

1974

(con. 1945) E. Thompson Tattoo (1977) 296: The boy’s buddy [...] said he was from ‘West-By-God-Virginia’.