How to respond to rude comments about name choice

My soon to be MIL had some choice rude words to say about what we are naming our son on thanksgiving.. it’s mainly my fiancé that wants the name but I also love it and it was my great x 2 grandfathers middle name. We’ve also had it picked since before we found out our daughter was a girl!

She would not stop saying how much she hated it and was making rude remarks the entire night being super childish about it. It doesn’t bother my partner at all! He’s just whatever about it. I hate confrontation and usually it’s hard for me to speak up but we are seeing her Sunday and I want to have the right thing to say because I know at some point it will be the topic of discussion..