expression
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.
百歩譲って、彼の言い分が正しいとしても、やり方は間違っている。
Even if we grant that his argument is correct, his approach is still wrong.
百歩譲ってその話が本当だとしても、今さらどうにもならない。
Even supposing that story is true, there's nothing we can do about it now.
たとえ is used with ～ても to mean 'even if', but 百歩譲って carries a stronger nuance of reluctantly granting an opponent's premise.
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.