Translation of unknown idiom from Spanish into English: I have come
across the expression: "a un traidor, dos alevosos." It is translated
as: "To bestry a traitor is not treason." But what does the Sapnish
word "alevosos" mean in English?

Dear sheila73,

In this context the word "alevosos" means "treacherous ones"
(singular: alevoso - the treacherous one; adjective: alevoso -
treacherous). So the full translation of the expression "A un traidor,
dos alevosos" would be: "For one traitor, there are two perfidious
ones" (I used a synonym for 'treacherous' here to keep the character
of the Spanish original, where also two unresembling words are
utilised).

A website with Spanish proverbs lists this saying with the following
explanation (translated from Spanish):

"A UN TRAIDOR, DOS ALEVOSOS - A proverb which means that someone who
works with treason does not deserve to enjoy fidelity."

Source:
Aforismos, Refranes y Locuciones - Refranes, T
http://www.ameracam.com/Marcos%20T/Refranes%20T.htm

Additional sources:

travlang's Translating Dictionaries
http://dictionaries.travlang.com/otherdicts.html

Google Language Tools
://www.google.com/language_tools?hl=en

Search terms used:
"a un traidor," alevosos
://www.google.de/search?hl=de&ie=UTF-8&oe=UTF-8&newwindow=1&q=%22a+un+traidor%2C%22+alevosos&btnG=Google-Suche&meta=

Hope this helps!
Best regards,
Scriptor

Thank you for such a speedy, exhaustive response. I cannot explain why
my dictionary have this information, but I am deligted to have it from
you!