suru verb (special), intransitive verb
Literary and somewhat old-fashioned; used for serving a person of high status, such as a lord or master. In modern Japanese, 仕える (つかえる) is more common.
彼は若い頃、大名に侍していた。
In his youth, he served a feudal lord.
仕える is the standard modern verb for serving a master or superior. 侍する is a literary, less common alternative with a similar meaning.
Derived from the noun 侍 (さむらい, warrior/attendant) with the verbal suffix する. The reading じする is a historical sound change from さむらいする.