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Related to gift horse: don't look a gift horse in the mouth
a gift horse
A present, often one that is flawed or unwanted. Often used in the saying "Don't look a gift horse in the mouth" (and similar variations), which is attributed to St. Jerome and refers to the practice of looking at a horse's teeth to determine its age. "Gift horse" may also be related to the Trojan Horse (a deceptive gift from the Greeks to the Trojans that allowed Greek soldiers access to Troy). I know Aunt Jean isn't your favorite person, but she gave you that beautiful sweater as a present, so don't look a gift horse in the mouth! A: "But I don't want this ancient car!" B: "Never look a gift horse in the mouth, OK? You're so lucky to get a car for free!"
Something obtained at no charge, but not without an ultimate cost. According to legend, it was Odysseus who devised the scheme of leaving a huge wooden horse in front of the gates of Troy, which the Greeks were unable to conquer (the losing side of a war traditionally left a gift for the victors). The Trojans watched the Greeks depart then dragged the horse inside their walls. Soldiers who had been hiding inside the horse surreptitiously opened the gates, the Greeks stole back inside the gates, and the rest is history. The cautionary expression “beware Greeks bearing gift” is based on the legend, as is the phrase “gift horse.” Stated another way, there ain't no free lunch