somebody or something that undermines an organization from within it
in computing, a program whose function is to cause some kind of damage to a system or to allow an unauthorized user to bypass a system's security, esp one that is disguised as ordinary software
so called because of the large hollow wooden horse which, in Greek mythology, was left by the Greeks outside Troy, which they were besieging. The Trojans took the wooden horse into the city, whereupon soldiers hidden inside it let the Greek army in