slack作名词表示 “(绳索的)松弛部分” ，这个单词作形容词有“松懈的”的意思，那cut someone some slack是“给……方便，对……网开一面，给……面子，放某人一马，放过某人一次”。

【例句】
Cut her some slack. She just had a baby.
对她网开一面吧。她刚生小孩。

If you cut someone some slack, you are less critical of their behaviour or performance than usual because you know they are in a difficult situation.
如果你放某人一马，你就不会像平常那样对他们的行为或表现那么挑剔，因为你知道他们正处于困境之中。