Calling Him a Samaritan was a derogatory way of denying Jesus is The Christ:
And here he is, speaking openly, and they say nothing to him! Can it be that the authorities really know that this is the Christ? But we know where this man comes from, and when the Christ appears, no one will know where he comes from.” (John 7:26-27 ESV)
Others said, “This is the Christ.” But some said, “Is the Christ to come from Galilee? Has not the Scripture said that the Christ comes from the offspring of David, and comes from Bethlehem, the village where David was?” (John 7:41-42)
The Christ would not come from Samaria or be a Samaritan. So in addition to the insult, being born of sexual immorality (8:41) and an illegitimate son of Abraham (He does not know His true father), the claim He is a Samaritan means He cannot be the Christ.
Jesus does respond, albeit obliquely:
Jesus answered, “I do not have a demon, but I honor my Father, and you dishonor me. (8:49)
He does know who is Father is.
The primary focus of the Jewish people was the human ancestry and birthplace of the Christ. While Matthew and Luke each affirm Jesus fulfilled these requirements, John explains the Christ was also the pre-existent I Am:
Are you greater than our father Abraham, who died? And the prophets died! Who do you make yourself out to be?” (8:53)...Jesus said to them, “Truly, truly, I say to you, before Abraham was, I am.” (8:58)
Which is the reason John wrote:
She said to him, “Yes, Lord; I believe that you are the Christ, the Son of God, who is coming into the world.” (11:27)
but these are written so that you may believe that Jesus is the Christ, the Son of God, and that by believing you may have life in his name. (20:31)