the forbidden degrees

the number of steps of descent from the same ancestor that bar two related people from marrying

禁止通婚的亲等

forbidden fruit

a thing that is desired all the more because it is not allowed

禁果

God (或 Heaven) forbid

used to express a fervent wish that something does not happen

上帝（天）不容（表达不希望发生某事的强烈愿望）

[with clause]God forbid that this should happen to anyone ever again.

上帝不容这类事在任何人身上重演。