beware of the greeks bearing gifts = fear the greeks bearing gifts
if rivals or enemies show apparent generosity or kindness, you should be suspicious of their motives – proverb
This proverb refers to the Trojan priest Laocoon's warning in Virgil's Aeneid : timeo Danaos et dona ferentes : in which he warns his countrymen against taking into their city the gigantic wooden horse that the Greeks have left behind on their apparent departure. The fall of Troy results from their failure to heed this warning.
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it is all greek to me
I can't understand it at all – informal
Greek meaning unintelligible language or gibberish is recorded from the 16th century. In Shakespeare's Julius Caesar, Casca, having noted that Cicero speaks Greek, adds 'for mine own part, it was Greek to me'.