Saxon

Saxon c.1300, from L.L. Saxonem (nom. Saxo), usually found in pl. Saxones, from P.Gmc. *sakhsan (Cf. O.E. Seaxe, O.H.G. Sahsun, Ger. Sachse "Saxon"), with a possible literal sense of "swordsmen" (Cf. O.E. seax, O.Fris., O.N. sax "knife, short sword, dagger," perhaps ultimately from PIE root of SAW (Cf. saw) (1)). 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 ....

OED helpfully points out that the correct Old English (with an uninflected plural) would be nimað eowre seax. For other national names that may have derived from characteristic tribal weapons, Cf. FRANK (Cf. Frank), LOMBARD (Cf. Lombard). Still in 20c. used by Celtic speakers to mean "an Englishman." In reference to the modern German state of Saxony (Ger. Sachsen, Fr. Saxe) it is attested from 1630s. Saxon is the source of the -sex in Essex, Sussex, etc. (Cf. Middlesex, from O.E. Middel-Seaxe "Middle Saxons"). Bede distinguished the Anglo-Saxons, who conquered much of southern Britain, from the Eealdesaxe "Old Saxons," who stayed in Germany.