It means you are absolutely sure. Darn is a slang term.
Not a darn penny! -JS
Darn jungle beast. Darn wild animal.
Darn it, Dang, etc.
it means darn, wretched, bloody, cursed.
Not a darn thing
It is a euphemism for the word damn.
This card is over 1,000 and could be worth 2000 if it was not for this darn economy
pretty darn effin precious
Dash it all: roughly means "Darn it!"
Darn means to repair a tear in fabric by knitting or sewing the edges together. It is also an old-fashioned mild swear word.