The story
From the arcade game Pac-Man (1980), in which the character being chased can sometimes turn round and eat its pursuers.
Etymology from Wiktionary, CC BY-SA 4.0.
Pac-Man defense
Today it means: A response to an attempted hostile takeover of a company, in which the target company, or a 'white knight' acting on its behalf, tries to turn the situation round by acquiring all or much of the other company. Full definition →