A mesne lord was a lord

Lord

Lord is a title with various meanings. It can denote a prince or a feudal superior . The title today is mostly used in connection with the peerage of the United Kingdom or its predecessor countries, although some users of the title do not themselves hold peerages, and use it 'by courtesy'...

 in the feudal system who had vassals who held land from him, but who was himself the vassal of a higher lord. A mesne lord did not hold land directly of the king

Monarch

A monarch is the person who heads a monarchy. This is a form of government in which a state or polity is ruled or controlled by an individual who typically inherits the throne by birth and occasionally rules for life or until abdication...

, that is to say he was not a tenant-in-chief

Tenant-in-chief

In medieval and early modern European society the term tenant-in-chief, sometimes vassal-in-chief, denoted the nobles who held their lands as tenants directly from king or territorial prince to whom they did homage, as opposed to holding them from another nobleman or senior member of the clergy....

. His subinfeudated

Subinfeudation

In English law, subinfeudation is the practice by which tenants, holding land under the king or other superior lord, carved out new and distinct tenures in their turn by sub-letting or alienating a part of their lands....

estate was called a "mesne estate". He was thus an intermediate or "middle" tenant, which status is reflected in the mediaeval French word mesne, in modern French moyen.