the.com/hookup

means A casual sexual or romantic encounter entered into without the expectation of commitment or a defined relationship.

from Rooted simply in 'hook' — the physical fastener used for connecting two things — which by the 19th century had spawned 'hook up' meaning to connect or join, as with equipment or machinery. The phrase drifted through electrical and mechanical senses (hooking up a wire, hooking up with a ride) before American slang bent it toward people: first meaning simply to meet up or team up with someone, then narrowing, over the 20th century, into its current romantic-but-noncommittal sense. The shift mirrors the word's own logic — things get connected, then just as easily disconnected.