“Not rendering evil for evil, or railing for railing: but contrariwise blessing; knowing that ye are thereunto called, that ye should inherit a blessing.”
1 Peter 3:9

Only the word of God can give such an instruction- he is training you to be like him; to function in and express the grace of God in all you do.
See what they did to Jesus:
“Some of them began to spit on him. They blindfolded him and kept hitting him with their fists and telling him, “Prophesy!” Even the servants took him and slapped him around.” Mark 14:65, ISV
And see what he said about them:
“Then said Jesus, Father, forgive them; for they know not what they do…” Luke 23:34
And see what he did for them:
“For Christ also hath once suffered for sins, the just for the unjust, that he might bring us to God…” 1 Peter 3:18
And now, he is training you to walk like he walked. Slanders and insults may come your way, you are not the first, Jesus was slandered and insulted also:
“And many of them said, He hath a devil, and is mad; why hear ye him?” John 10:20
He was nearly stoned, many times: John 8:59, 10:31
Yet, he loved them and he wants your love to abound more and more. So don’t “give it back” to those who used saucy language towards you- instead, express the grace of God; bless them.