Re: Silly first name.
"It was a term used by the Highland clans about anybody"
Be that as it may, the root of the word is from the gaelic for "Saxon" and it means "English".
Which has lead to one of the more amusing new Irish words in recent years, "Sasamach" (Brexit) - which sounds almost identical to "Sasanach" (English) but is also portmanteau of "Sasana" (England) and "Amach" (out).