expression
not one; not a single one
Used with a negative verb to emphasize that absolutely none of the items in question applies. Often implies 'not even one' or 'not a single thing'.
彼の言うことはどれ一つ信用できない。
I can't trust a single thing he says.
この店の商品はどれ一つとして満足できるものがない。
There isn't a single product in this store that I find satisfactory.