In our discussion of the word shelet שלט, we said that Targum Yonatan on Divrei Hayamim translated שלטים as "shields". The word used in his translation is תריסין - terisin, or in the singular, tris תריס.
Klein writes that tris as shield comes from the Greek thyreos, meaning shield, which in turn derives from the Greek thyra - "door". (Going back to the Indo-European root, Klein shows that thyra is cognate with the English word "door" as well.) However, the connection between thyreos and thyra is strange to me. He writes that thyreos is a "stone put against the door". I don't see how that means shield.
I have an easier time understanding the entry in the Online Etymology Dictionary for "thyroid", which also derives from thyreos:
1690s (in ref. to both the cartilage and the gland), from Gk. thyreoiedes "shield-shaped" (in khondros thyreoiedes "shield-shaped cartilage," used by Galen to describe the "Adam's apple" in the throat), from thyreos "oblong, door-shaped shield" (from thyra "door") + -eides "form, shape." The noun, short for thyroid gland, is recorded from 1849.The Hebrew word for thyroid reflects this origin as well - בלוטת התריס balutat hatris.
We also have a verb that derives from tris - התריס - "to contradict, oppose". Klein is not clear about which sense - shield or shutter - led to this verb. First he has an entry for התריס meaning "he shielded, protected", and then figuratively meaning "he protested against, contradicted, debated." His second entry for התריס is defined as "to contradict, oppose", but the etymology is given as "denominated from תריס (=shutter)".
In his dictionary, Even Shoshan agrees with the former, and says it derives from shield, but doesn't explain why. He does, however, say that the great debaters known as baalei terisin בעלי תריסין (mentioned in Berachot 27b), knew how to argue in the "wars of Torah". (According to Rashi; the Aruch says they were literally soldiers). So perhaps this military imagery - Jastrow calls them the "shield bearers" - led to the connection between tris as shield and hitris התריס as "to contradict, oppose".
Yaakov Etsion in this article suggests that perhaps the development went like this: these were people who were willing to shield and defend themselves, and not give in to others. From here the concept progressed to those who took the offensive, and contradicted others when necessary.
Perhaps therefore this is an early version of the adage "The best defense is a good offense". But as comedian Norm Macdonald noted, "The second best defense is a good defense"...