watch one's back

撑腰、罩着某人

►含义：

It means to be willing and ready to help and protect someone if they are in need.

"watch one's back"是指当某人需要的情况下，愿意并时刻准好去提供帮助和保护。跟我们常说的“为某人撑腰”、“罩着某人”意思一致。

►例句：

The kid is afraid of nobody with his mother watching his back.

这个孩子仗着他妈妈给他撑腰，谁都不怕。

►对话：

A: You'd better tone it down and not mess with those guys out here. Nobody watches your back on their turf.

你最好老实点，在那不要招惹那帮家伙。在他们的地盘上没人会罩着你。

B: Got it.

记住了。

►额外收获： 

1.tone it down: 低调，收敛

2.mess with: 招惹

3.on one's turf: 在某人的地盘上