Literal Translation
It doesn’t break three legs on a duck
Actual Meaning
It’s nothing spectacular / it’s banal. Nothing about harming waterfowl. “It’s nothing to write home about”
Etymology
I had to turn to Expressio which normally has the answer but this time, seems to be at a loss as to the origin. It settles for this explanation: “Given the number of three-legged ducks that exist in the universe (and beyond), managing to break all the legs of one of them would be a miraculous feat. Not breaking all three legs of a duck, therefore, means being like everyone else or extremely ordinary.” Certainly.
Another, more colourful theory dates back to a time when the term ‘duck’ referred to something other than a feathered waterfowl. Knock-kneed horses, otherwise known as ‘cagnards’, may have played a role. These poorly built and clumsy horses were the laughing stock of riders. It was said that such a horse, with its joints turned inwards, was good at breaking its rider’s legs (and vice versa). The idea may have spread from the stables to the ponds, with the word cagnard becoming canard, perhaps out of a love of puns. Thus, a three-legged horse or duck symbolises the improbable.