The phrase "a snake in the grass" is commonly used to describe someone who is deceitful and cannot be trusted. There are many synonyms for this phrase that can be used in different contexts. One simple alternative is "deceitful," which can be used to describe a person who is deliberately dishonest. "Duplicitous" is another synonym that is commonly used to describe someone who is two-faced and unreliable. "Fraudulent" is another word that can be used to describe someone who is dishonest and deceiving. Other synonyms include "treacherous," "sneaky," and "untrustworthy." No matter which word is used, the meaning is clear that someone is not to be trusted and should be approached with caution.