A more thorough explanation:

An ancient serjeant is a term used in English law to refer to the eldest of the Crown's serjeants. This position was held by the last serjeant who died in 1866.

For example, if the Crown had multiple serjeants, the ancient serjeant would be the one with the most seniority and experience.

This term is not commonly used today, but it is important to understand its historical significance in English law.