Refers to one's feudal lord or master in a historical samurai context; not used for modern bosses or landlords.
The samurai swore loyalty to his lord.
He could not disobey his master's orders.
主君 means the lord or master whom a 臣 (retainer) serves, making it the natural opposite in a feudal relationship.
主君 is the lord a 家来 serves, making the two words opposite roles in a feudal relationship.
主君 is the lord a 家臣 serves, making the two words opposite ends of the feudal relationship.
Compound of 主 (しゅ, 'lord, master') and 君 (くん, 'ruler, lord'). The reading is on'yomi. The term is firmly associated with the Edo-period samurai class.