The origin of the office of capellanus or cappellanus in the medieval church is generally traced (see Du Cange, Gloss.
However this may be, the name capellanus was generally applied to those who were in charge of sacred relics: such officials were also known as custodes, martyrarii, cubicularii.
From the care of sacred relics preserved in royal chapels, &c. (sacella or capellae), the office of capellanus naturally extended its scope until it covered practically that of the modern court chaplain, and was officially recognized by the Church.
Such a sanctuary was served by a priest, who was hence called capellanus, from which is derived the English "chaplain" (q.v.).