Why so sardonic, Sardinians?

I like to think I have a pretty good handle on the origins
of common phrases. I like to think wrong, as I learned from The Etymologicon: A Circular Stroll through the Hidden Connections of the English Language by Mark Forsyth.
To wit:

Sardonic– Referred to people from Sardinia who, in ancient
times, were thought of as unfriendly.
Bite the dust– A direct translation of a quote from The Iliad in which a character talks of
the death of Hector, who was dragged around behind a chariot.
Humble pie– A meal made using the "umbles" (guts) of deer
and eaten by the lowly servants.
Nazi– An insult in use long before the rise of Adolf
Hitler's party. It was a derogatory term for a backwards peasant, being a
shortened version of Ignatius, a common name in Bavaria, the area from which
the Nazis emerged. Opponents seized on this and shortened the party's title
Nationalsozialistische Deutsche Arbeiterpartei, to the dismissive
"Nazi".
In the doghouse– From J.M. Barrie's 1911 novel Peter Pan, in
which Mr. Darling forces the dog to sleep in the kennel, and as a result the children
disappear. As penance, Mr. Darling takes to sleeping in the doghouse himself.
Hoax– From "hocus-pocus", which was used by Protestants to
ridicule the rite of consecration carried out in the course of Catholic mass,
which includes the phrase "Hoc est
corpus meum" ("This is my body").
Bigot– Old English for "by god", to describe
someone who asserts their own saintliness, while being a hypocrite.

Via.