In contrast to many other idioms that have their origins in times long past, the explanation for "A gift horse is not to be looked into the mouth" can even be transferred to the present. Horse buyers check the age of the animals by examining their teeth.
Explanation for "You don't look into a gift horse's mouth"
Horses, like humans, have molars and incisors. And these wear out over time - especially the molars are rubbed off over time by grinding the food. A buyer can therefore tell from the degree of bit wear on the animal whether the seller has cheated a little about the real age of the horse. However, if the horse is a gift, it would be rude to take a closer look at the age, so the explanation.
The saying with the gift horse, which should not be looked into the mouth, exists in all possible cultures and has its origin in the Latin "Noli equi dentes inspicere donati". In a figurative sense, it is said that gifts should be accepted joyfully and gratefully and neither question their material value nor criticize them in any way.
What do I do with my gift horse?
You have explicitly requested something very special from your loved ones for Christmas or a birthday; but a look at the gift table reveals: It hasn't quite turned out as you imagined it. Or does your mother-in-law consider you useless decorative items that do not suit your taste at all possible occasions? Of course you know that you shouldn't be ungrateful, after all you don't look into a gift horse's mouth. So what to do?
Psychological and diplomatic sensitivity is required, because the person giving the gift probably meant it well from his perspective. If you then appear to be obviously disappointed, offended or upset because you personally misunderstand the gift that is inappropriate, in the worst case you may feel patronized, the crooked hanging blessing is inevitable. You should only react with honesty if you do not care about a harmonious social relationship with the donor or if you know for certain that he wanted to annoy you. Otherwise, it's best to lie and pretend that you like the hideousness.
Or you try it with a friendly half-truth, say, for example, that you already have the boring book or that the ugly sweater doesn't match the color of your wardrobe, but is otherwise very beautiful. This way, with a little luck, you can get the receipt and exchange the inappropriate gift. However, there is no general right of exchange if the goods are unused and in perfect working order. Most sellers take back unloved gifts out of goodwill, especially if the customer asks nice and friendly.