siesed thereof in his desmense as of fee

In the feudal system, demesne; via Old French demeine from Latin dominium)^[1] was all the land, not necessarily all contiguous to the manor house, that was retained by a lord for his own use - as distinguished from land "alienated" or granted to others (alieni) as freehold tenants.

Seisin primarily means possession, and for several centuries after the Conquest it was the only word known to the English lawyers capable of conveying this meaning. It was consequently applied at one time to the possession of chattels, as well as of land. Later it was applied only to the possession of land or of incorporeal things, and in this connection it came finally to be used only in reference to possession by one claiming a freehold estate; he being said to be "seised," while a tenant for years or at will was said to be merely "possessed."

The word, while suggestive to our minds, from its similarity to the word "seize," of the idea of violence, is in reality only distantly connected with the latter word, and is to be associated rather with the words to "sit" and to "set," with which it is also connected etymologically, and properly implies the idea of one being "set" on land, and thereafter sitting there in rest and quiet.