This quote is from the second book of the Aeneid, when Lacoon, a priest, warns the Trojans not to accept the giant wooden horse that has apparently been left by the Greeks. Another translation is "Trust not their presents, nor admit the horse." Of course, Laocoon turns out to be right, though it is difficult to see this at the time, as Poseidon sends a giant serpent to strangle him in front of the Trojans' astonished eyes. The Trojans took in the horse, which was filled with Greek warriors led by the cunning Odysseus, and it led to their destruction. In terms of lessons for today, the phrase might be a warning not to be swayed by gifts, or even flattery. It could also have another connotation. If things seem too good to be true, they probably are. The Trojans believed that the Greeks had left after years of warfare because they desperately wanted to think this was possible, and that the war would be over. They did not believe Laocoon's eminently sensible warning because they so desperately wanted to believe otherwise.