(exclaimation) Oldcootism used during great consternation or surprise. Used by 1890’s prospectors, cantankerous old farmers, and young people playing old people on TV in the ‘60’s and ‘70’s.
Them dern aliens ruined my golldarn rubbarb patch with their flyin' machine, dagnabbit!
When one gets upset at something or someone than decides not to take "the man upstairs" name in vain therefore blurting out "Dag nammit". Also used by midwest rednecks.
Dag nammit Girtrude you done did it again and broke my dang ol' rifle, now how I'm going to catch me some possum?
Its just a less blasphemous way of saying God dammit, it means exactly the same thing because it is the same thing. Its a bit like cockney rhyming slang.
Why are you all unable to see that this means God dammit!? Dag nammit!