Etymology 1

From the economics of horse ownership, in which an aging horse of declining value continues to require feeding and stabling.

Noun

1. An apparent gift, that has substantial associated costs.
   • 1837, Washington Irving, The Adventures of Captain Bonneville,

     Having now, as he thought, balanced this little account of friendship, the captain was about to shift his saddle to this noble gift-horse when the affectionate patriarch plucked him by the sleeve, and introduced to him a whimpering, whining, leathern-skinned old squaw, that might have passed for an Egyptian mummy, without drying. "This," said he, "is my wife; she is a good wife--I love her very much.--She loves the horse--she loves him a great deal--she will cry very much at losing him.--I do not know how I shall comfort her--and that makes my heart very sore."

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Noun

1. gift which one is expected to accept without complaint or criticism.
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Usage notes

Looking in the mouth of a horse is a common way to assess its age, condition and therefore the value. As a horse is always worth something, there is no need to check a free horse to see if you're getting it for a good price...