"Look a (gift) horse in the mouth"

[W Brewer]

WB:  By now, about the only thing I remember from my Hittite classes was
that they would leave diseased livestock before an enemy city, whose
denizens would take them in and become weakened prey for the Hittites.
Equines & ovines as biological weapon c. 2500 BC; subsequent Trojan Horse =
metaphor for germ warfare. (In olden days, I would have jumped up and
translated "Never look a gift sheep in the mouth" into Hittite. Never mind.
Wait for Google translator to catch up.)

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