Enemigo que hace regalos, seguro que quiere algo

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Meaning:

We use this expression as a warning. We tell others to be suspicious of presents from those who are not our friends or from those with whom we have had problems with in the past.

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Origin:

The Roman writer, Virgil,  wrote the Aeneid. In it, the Greek army tried for ten years to conquer the city of Troy.  In the end, the Greeks played the Trojans a trick. They pretended to leave away and left behind  a large wooden horse as a gift. In it, hundreds of Greek soldiers were hidden because the belly of the horse was hollow. When the Trojans pulled that horse into the city, the Greeks went out at night and defeated the Trojans, conquering the city as a consequence.

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Example:

Google images, «Beware of Greeks bearing gifts» Click on the image to hear a song by Tantrumsanthem