He's a snake in the grass. While pretending to be your friend he was slandering you behind your back.

他是个暗敌，表面上装作是你的朋友，背地里却在诽谤你。

《新英汉大辞典》

Aren't I a snake in the grass?

我是不是草丛里的一条蛇呀？

youdao

Aren't I a snake in the grass.

我不就是草丛里的一条蛇。

youdao

It is no coincidence that the next snake in the grass to go nuclear with North Korea's help may well be Burmese.

CENTERFORSECURITYPOLICY: Snakes in the grass

After all, another sort of snake in the grass from Burma is currently making its presence known on the world stage for the first time in years, thanks to the Obama administration and its emissary in fact, if not in name: U.S. Senator Jim Webb of Virginia.

CENTERFORSECURITYPOLICY: Snakes in the grass

The RSPCA was called back to Baxenden and District Golf Club, near Accrington, on Wednesday, a day after the first snake was caught in the long grass.

BBC: Second boa constricts golfers