Is it true lawyers are the biggest liar?

Lawyers certainly have to stretch the truth a lot at times. As an officer of the court lawyers are less likely and less often going to lie while in court session but outside the court is a different story.

There are rules and ethical standards that lawyers have to live by or else they risk having their license to practice law suspended or get disbarred. If lying in a certain situation doesn't violate any rules or standards of practice and the lying would help the lawyer's client the lawyer is obligated as the client's advocate to do so.