nobleman vs mesne lord

nobleman and mesne lord both are nouns.

Word	Noun	Adjective	Verb	Adverb
nobleman	Yes	No	No	No
mesne lord	Yes	No	No	No

As nouns, mesne lord is a hyponym of nobleman; that is, mesne lord is a word with a more specific, narrower meaning than nobleman:

nobleman: a titled peer of the realm
mesne lord: a feudal lord who was lord to his own tenants on land held from a superior lord

nobleman (noun)	mesne lord (noun)
a titled peer of the realm	a feudal lord who was lord to his own tenants on land held from a superior lord