arm (n.2)youdaoicibaDictYouDict[arm 词源字典]
"weapon," c. 1300, armes (plural) "weapons of a warrior," from Old French armes (plural), "arms, war, warfare," mid-13c., from Latin arma "weapons" (including armor), literally "tools, implements (of war)," from PIE root *ar- "fit, join" (see arm (n.1)). The notion seems to be "that which is fitted together." Meaning "heraldic insignia" (in coat of arms, etc.) is early 14c.; originally they were borne on shields of fully armed knights or barons.[arm etymology, arm origin, 英语词源]
estate (n.)youdaoicibaDictYouDict
early 13c., "rank, standing, condition," from Anglo-French astat, Old French estat "state, position, condition, health, status, legal estate" (13c., Modern French état), from Latin status "state or condition, position, place; social position of the aristocracy," from PIE root *sta- "to stand" (see stet).

For the excrescent e-, see e-. Sense of "property" is late 14c., from that of "worldly prosperity;" specific application to "landed property" (usually of large extent) is first recorded in American English 1620s. A native word for this was Middle English ethel (Old English æðel) "ancestral land or estate, patrimony." Meaning "collective assets of a dead person or debtor" is from 1830.

The three estates (in Sweden and Aragon, four) conceived as orders in the body politic date from late 14c. In France, they are the clergy, nobles, and townsmen; in England, originally the clergy, barons, and commons, later Lords Spiritual, Lords Temporal, and commons. For Fourth Estate see four.
Saxon (n.)youdaoicibaDictYouDict
c. 1200, from Late Latin Saxonem (nominative Saxo; also source of French Saxon, Spanish Sajon, Italian Sassone), usually found in plural Saxones, from a Germanic source (Old English Seaxe, Old High German Sahsun, German Sachse "Saxon"), with a possible literal sense of "swordsmen" (compare Old English seax, Old Frisian, Old Norse sax "knife, short sword, dagger," Old High German Saxnot, name of a war-god), from Proto-Germanic *sahsam "knife," from PIE *sek- "to cut" (see section (n.)).

The word figures in the well-known story, related by Geoffrey of Monmouth, who got it from Nennius, of the treacherous slaughter by the Anglo-Saxons of their British hosts:
Accordingly they all met at the time and place appointed, and began to treat of peace; and when a fit opportunity offered for executing his villany, Hengist cried out, "Nemet oure Saxas," and the same instant seized Vortigern, and held him by his cloak. The Saxons, upon the signal given, drew their daggers, and falling upon the princes, who little suspected any such design, assassinated them to the number of four hundred and sixty barons and consuls ....
The OED editors helpfully point out that the correct Old English (with an uninflected plural) would be nimað eowre seax. For other Germanic national names that may have derived from characteristic tribal weapons, see Frank, Lombard. As an adjective from 1560s. Still in 20c. used by Celtic speakers to mean "an Englishman" (Welsh Sais, plural Seison "an Englishman;" Seisoneg "English").

In reference to the modern German state of Saxony (German Sachsen, French Saxe) it is attested from 1630s. Saxon is the source of the -sex in Essex, Sussex, etc. (compare Middlesex, from Old English Middel-Seaxe "Middle Saxons"). Bede distinguished the Anglo-Saxons, who conquered much of southern Britain, from the Ealdesaxe "Old Saxons," who stayed in Germany.