M-W thinks that

Probably the best fancy way to describe a liar is mendacious. It comes from the Latin word mendax, meaning “lying” or “false.” It is often used to refer to people who habitually lie.

More specifically, the definition is

given to or characterized by deception or falsehood or divergence from absolute truth (M-W)

I must say I also like dissembler, which, though somewhat dated, seems to pick up in our century:

a person who professes beliefs and opinions that he or she does not hold in order to conceal his or her real feelings or motives (FreeDictionary)

M-W defines the verb to dissemble as:

to put on a false appearance : conceal facts, intentions, or feelings under some pretense

M-W thinks that

Probably the best fancy way to describe a liar is mendacious. It comes from the Latin word mendax, meaning “lying” or “false.” It is often used to refer to people who habitually lie.

More specifically, the definition is

given to or characterized by deception or falsehood or divergence from absolute truth (M-W)

I must say I also like dissembler, which, though somewhat dated, seems to pick up in our century:

a person who professes beliefs and opinions that he or she does not hold in order to conceal his or her real feelings or motives (FreeDictionary)

M-W defines the verb to dissemble as:

to put on a false appearance : conceal facts, intentions, or feelings under some pretense