When someone does ninda of you, they take your paap on their head. There's a sakhi about Bhagat Kabir ji that illustrates this. Kabir ji used to have a neighbor who hated him (or was jealous of him!). The neighbor used to say a lot of wrong things about Bhagat Ji. One day the neighbor died. Kabir ji looked very sad at this news. People asked him, "why are you sad? you should be happy, your neighbor who did your ninda is dead." Kabir ji replied, "I was happy when he did ninda, he took my sins upon his head. now who will wash all of my sins?"

Here are the lines from gurbani:

ਗਉੜੀ ॥
Gauree:

ਨਿੰਦਉ ਨਿੰਦਉ ਮੋ ਕਉ ਲੋਗੁ ਨਿੰਦਉ ॥
Slander me, slander me - go ahead, people, and slander me.

ਨਿੰਦਾ ਜਨ ਕਉ ਖਰੀ ਪਿਆਰੀ ॥
Slander is pleasing to the Lord's humble servant.

ਨਿੰਦਾ ਬਾਪੁ ਨਿੰਦਾ ਮਹਤਾਰੀ ॥੧॥ ਰਹਾਉ ॥
Slander is my father, slander is my mother. ||1||Pause||

ਨਿੰਦਾ ਹੋਇ ਤ ਬੈਕੁੰਠਿ ਜਾਈਐ ॥
If I am slandered, I go to heaven;

ਨਾਮੁ ਪਦਾਰਥੁ ਮਨਹਿ ਬਸਾਈਐ ॥
the wealth of the Naam, the Name of the Lord, abides within my mind.

ਰਿਦੈ ਸੁਧ ਜਉ ਨਿੰਦਾ ਹੋਇ ॥
If my heart is pure, and I am slandered,

ਹਮਰੇ ਕਪਰੇ ਨਿੰਦਕੁ ਧੋਇ ॥੧॥
then the slanderer washes my clothes. ||1||

ਨਿੰਦਾ ਕਰੈ ਸੁ ਹਮਰਾ ਮੀਤੁ ॥
One who slanders me is my friend;

ਨਿੰਦਕ ਮਾਹਿ ਹਮਾਰਾ ਚੀਤੁ ॥
the slanderer is in my thoughts.

ਨਿੰਦਕੁ ਸੋ ਜੋ ਨਿੰਦਾ ਹੋਰੈ ॥
The slanderer is the one who prevents me from being slandered.

ਹਮਰਾ ਜੀਵਨੁ ਨਿੰਦਕੁ ਲੋਰੈ ॥੨॥
The slanderer wishes me long life. ||2||

ਨਿੰਦਾ ਹਮਰੀ ਪ੍ਰੇਮ ਪਿਆਰੁ ॥
I have love and affection for the slanderer.

ਨਿੰਦਾ ਹਮਰਾ ਕਰੈ ਉਧਾਰੁ ॥
Slander is my salvation.

ਜਨ ਕਬੀਰ ਕਉ ਨਿੰਦਾ ਸਾਰੁ ॥
Slander is the best thing for servant Kabeer.

ਨਿੰਦਕੁ ਡੂਬਾ ਹਮ ਉਤਰੇ ਪਾਰਿ ॥੩॥੨੦॥੭੧॥
The slanderer is drowned, while I am carried across. ||3||20||71||