거짓말쟁이 A쟁이 means "a person who does A" so, in this case it will be "a person who tell lies" this is a formal and not so intense but, you also need to know "구라쟁이" yeah, it would be the equivalent for "phony" or whatnot. the first part "구라" sounds "Goo-Ra" and means the same as "거짓말" and the rest is what you already know :D