Liaison

French liaison comes from Latin ligare, 'to tie' or 'to bind.' A liaison was literally something that bound two things—an attachment, a connection, a bond. In medieval warfare, a liaison officer was the person who bound two army units together by carrying messages between them.

In French cooking, a liaison is the egg-and-cream mixture that binds a sauce—the emulsion that holds disparate elements into one cohesive whole. This is the cuisine meaning, still used: you bind the sauce with a liaison. The metaphor is perfect: the liaison is what makes separated things cohere.

By the 1700s, English borrowed liaison for both military communication and romantic entanglement. A liaison also meant an illicit relationship—something binding two people who shouldn't be bound. The word carried the cooking metaphor: the affair was the binding agent between two separate lives.

Modern English uses liaison for any coordinating relationship: liaison officer, community liaison, press liaison. The romance and secrecy have faded. We're back to the original meaning: something that binds things together. The word has cycles—it moves from concrete to metaphorical to scandal to bureaucratic and back again.