【俚语】

Back off 退后

【例句】

Back off！Don’t touch me！
后退！别碰我！

Step back 退后

【例句】

Please step back from the injured woman. Give her some air.
不要靠近受伤的女士，请往后退一点，让她透透气。

Keep back! 退后

【例句】

"Keep back!" he shouted, "Don't come any closer!”
“退后！”他大叫着，“不要再靠近了！”

Back and forth
反复地；来回地

【例句】

During the party, I went back and forth to the kitchen to get drinks for the guests.
在聚会上，我来来回回从厨房给客人们拿饮料。

Back in the day
曾几何时

【例句】

Back in the day, I used to spend my summers exploring or playing with friends. Now I'm in the office every day.
曾几何时，我常常在夏天和朋友们玩或者去野外探索。现在我每天都只能在办公室。