The phrase is an exclamation used when nothing else will fit. Often fitting when one is stunned or dismayed.
The Oxford English Dictionary has just one example, from — of all sources — James Joyce’s Ulysses: “Paddy Leonard eyed his alemates. Lord love a duck, he said. Look at what I’m standing drinks to!” But T S Eliot also used it, in The Rock of 1934: “Lor-love-a-duck, it’s the missus!”.
It also turns up a number of times in the works of P G Wodehouse, the earliest being The Coming of Bill, two years before Ulysses was published: “‘Well, Lord love a duck!’ replied the butler, who in his moments of relaxation was addicted to homely expletives of the lower London type.”
It was originally British, though it has since emigrated to other Commonwealth countries. And that origin is supported by the earliest example I’ve found, in a long-forgotten tale of 1907, The Wheel O’ Fortune, by Louis Tracy, a British journalist and prolific author: “‘Lord love a duck!’ he guffawed. ‘If only I’d ha’ knowed, I could have told my missus. It would have cheered her up for a week.’”
It’s a mild and inoffensive expression of surprise, once well known in Britain and dating from the latter years of the nineteenth century.
http://www.worldwidewords.org/qa/qa-lor1.htm
http://www.usingenglish.com/reference/idioms/lord+love+a+duck.html
