a snake in the grass可不是“草丛中的蛇”，真实含义是阴险的人，潜伏的敌人，虚假的人。

【例句】
Mike is a snake in the grass. While pretending to be your friend he was slandering you behind your back.
迈克是个阴险的人，表面上装作是你的朋友，背地里却在诽谤你。

I always thought John a true friend, but his action later proved him to be a snake in the grass.
我过去一直认为约翰是个真正的朋友，但他后来的行为证明他是个奸诈的人。