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.
by Stating the bleedin obvious. March 24, 2018
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?
by Doogie Howser M.D. February 20, 2009