noun
Refers to one's own master, employer, or lord in a hierarchical relationship. Often used in historical or formal contexts, or when speaking humbly about one's superior. The reading しゅう is less common than しゅ for this meaning.
See also: 主 (しゅ)
彼は主に忠誠を誓った。
He swore loyalty to his lord.
主のご意向を伺ってまいります。
I will go and ask my master's intentions.
主人 is the standard word for 'master' or 'employer' in modern Japanese, while 主 (しゅう) is more formal or archaic.
The reading しゅう is a variant of the more common しゅ, both derived from Middle Chinese. The exact historical development of this reading is uncertain.