E. Cobham Brewer 1810–1897. Dictionary of Phrase and Fable. 1898.
Thessalian.
Deceitful, fraudulent; hence Θεo = fraud or deceit. Θεo = double dealing, referring to the double-dealing of the Thessalians with their confederates, a notable instance of which occurred in the Peloponnesian War where, in the very midst of the battle, they turned sides, deserting the Athenians and going over to the Lacedæmonians. The Locrians had a similar bad repute, whence Λoυ; but of all people, the Spartans were most noted for treachery.
