Lord love a duck!

I have relatives who on occasion exclaim "Lord love a duck!" But I'm not sure what is meant by it. Does anyone know this one?

Also, does anyone have any idea where it comes from?

It is an exclamation in the "boy, oh, boy" or "wowee" vein.
I have no idea as to it's derivation.

There was a 1966 or so film starring Roddy McDowall, Tuesday Weld, Lola Albright, Ruth Gordon and Harvey Korman named "Lord Love A Duck", a way out off-beat comedy that lampooned the 60s Southern California teen culture.

Is it - or can it be - positive, negative, or neutral?

I've always heard it as mild amazement, expressing much the same sentiment as "well now I've seen everything," or "imagine that!"