What is Serjeant?

Legal Definition
The same word etymologically with "sergeant," but the latter spelling is more commonly employed in the designation of military and police officers, (see Sergeant,) whlle the former ls preferred when the term is used to describe cortain grades of legal practitioners and cortain officers of legislative bodies. See infra.
See also
-- Black's Law Dictionary
Legal Definition
The highest rank attainable in the profession of the common law.
-- Ballentine's Law Dictionary