anna, this word is in my backlog of potential wwftd[s]; here is what I have on sassenach:

("s&s@n&x) [repr. Gael. Sasunnach adj. English, n. an Englishman = Irish Sasanach, Sacsanach, f. Sasan-, repr. the Teut. ethnic name Saxon. (Cf. Gael. Sasunn, Irish Sasana, Sacsain, England.)]

The name given by the Gaelic inhabitants of Great Britain and Ireland to their ‘Saxon’ or English neighbours. (Sometimes attributed to Welsh speakers: the corresponding Welsh form is Seisnig.)

All loved their McClan, save a Sassenach brute, Who came to the Highlands to fish and to shoot. - 1869 W. S. Gilbert (Bab Ball. 187)