Why is it called Hollow Knight?

Why is it called Hollow Knight? A few Vessels are shown to have escaped the Abyss, one being the Knight. Eventually, The Pale King chose the most suitable Vessel, deemed the “Hollow Knight”, and used it to contain both the Infection and the Radiance.

A few Vessels are shown to have escaped the Abyss, one being the Knight. Eventually, The Pale King chose the most suitable Vessel, deemed the “Hollow Knight”, and used it to contain both the Infection and the Radiance.