What is a Sassenach?

"Sassenach is a word used chiefly by the Scots to designate an Englishman"

https://en.wikipedia.org/wiki/Sassenach

Pronunciation: (sas'u-nukh, -nak), [key] —n.

An English inhabitant of the British Isles: used, often disparagingly, by the Gaelic inhabitants.

http://dictionary.factmonster.com/sassenach

Welsh has a similar word: "Saesneg" (= "English").

https://en.wiktionary.org/wiki/Saesneg

Literally, means Saxon. ("There are two words in modern Welsh for the English and this reflects the idea held by some that the modern English derive from various Germanic tribes (although there is little evidence for the extinction of the pre-Germanic inhabitants of England, and the idea ignores both the Scandinavian settlers in England and the Roman and Norman-French influences on English language, culture and identity): Saeson (singular: Sais), meaning originally Saxon; and: Eingl, denoting:-Angles,; meaning Englishmen in modern Welsh. The Welsh word for the English language is Saesneg, while the Welsh word for England is Lloegr." https://en.wikipedia.org/wiki/Welsh_people ) Whereas the Saxons just called the aboriginal inhabitants 'foreigners'....