To have been (usually unintentionally) struck very hard in the sensitive groin region, resulting in a large impact force upon the testicles. Symptoms include extreme pain between legs and doubling over, to being floored (if the impact is enough).
Nadding is highly unpleasant. Any impact which does not draw a pained reaction of some kind was not a proper nadding, but probably a near miss or was simply too light to be considered a nadding.
An intentional strike upon the sensitive area is known as a low blow