Fig.: said of a hidden or hypocritical enemy

Don't trust that guy. Everybody knows that he's a sneaky snake in the grass

Non fidarti di quel tipo. Lo sanno tutti che è una viscida serpe in seno

I trusted him dearly. He was an old family-friend, but he turned out to be a snake in the grass when he followed only his own interest and didn't care about his old friends

Mi fidavo di lui, era un vecchio amico di famiglia, ma si è rivelato una serpe in seno quando ha pensato solo al proprio interesse, fregandosene dei vecchi amici

This methaphor, which alludes to a poisonous snake hidden in tall grass, was first used by the Roman poet Virgil in 37 B.C. in his Eclogue III, v. 93: Latet anguis in herba. It was first recorded in English in 1696, as the title of a book by Charles Leslie