Trojan horse

Tro′jan horse′,

Mythology[Class. Myth.]a gigantic hollow wooden horse, left by the Greeks upon their pretended abandonment of the siege of Troy. The Trojans took it into Troy and Greek soldiers concealed in the horse opened the gates to the Greek army at night and conquered the city.
a person or thing intended to undermine or destroy from within.
Computinga nonreplicating computer program planted illegally in another program to do damage locally when the software is activated.

Collins Concise English Dictionary © HarperCollins Publishers::

Trojan Horse n

Also called: the Wooden Horse the huge wooden hollow figure of a horse left outside Troy by the Greeks when they feigned retreat and dragged inside by the Trojans. The men concealed inside it opened the city to the final Greek assault
a trap intended to undermine an enemy
a bug inserted into a program or system designed to be activated after a certain time or a certain number of operations

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