I've always thought that exclamations like "fiddlesticks" and "shoot!" were substitutions for other, more vulgar, oaths such as f--k and s--t. Likewise darn for damn, heck for hell, gosh for God, etc. They don't have to make any sense, as they're just substitute sounds, such as when "Doctor Evil" in the Austin Powers films says, "Throw me a frickin' bone here, people!"