At one time the highest rank of English barrister at the bar; it was abolished in the late 19c. Working in the court of common pleas, the serjeants-at-law had a monopoly on pleading cases. They were appointed by the king after 16 years of study and practice. Justices of the court of common pleas were chosen from their ranks. The title is a corrupt version of serviens ad legem, i.e. a law servant. They were serjeant-counters, i.e. pleaders who would frame the counts or charges involved in a case. Though similar in origin to 'sergeant', the spelling today of the word when referring to a law officer always uses a 'j'; however, use of a 'j' does not necessarily signify a law officer. In Chaucer's lifetime there were rarely more than 20 such officers. -
Dictionary of Medieval Terms and Phrases. Christopher Coredon with Ann Williams.