let someone go ahead by 100 steps
(I don’t accept/agree with your behaviour/opinion even if) I make a lot of concessions and grudgingly accept some of your point of view
It’s usually used when you state that something is utterly unacceptable no matter what. It’s commonly used as “百歩譲って…としても” (“Even if I make a lot of concessions and …”). To put more emphasis on unwillingness, you can also say 一万歩譲って/百万歩譲って, etc.
My boyfriend forgot my birthday, and even if I were to grudgingly accept it as an “oh well, sh*t happens” thing, it was utterly intolerable that the birthday present he gave me was a f*cking bookshop gift card.