for the sake of argument; even if that were true
Used as a concessive phrase meaning 'even if I grant you that much' or 'even supposing that were the case'. Often appears at the beginning of a sentence to introduce a hypothetical concession before making a counterpoint.
Even if we grant that his argument is correct, his approach is still wrong.
Even supposing that story is true, there's nothing we can do about it now.
Literally 'conceding a hundred steps', from the metaphor of giving ground in an argument. The exact historical origin is uncertain, but it is a common set phrase in modern Japanese.