The story
From code + share, because of the change in the flight's two-letter airline identity code. One aircraft will have two or more codes and flight numbers.
Etymology from Wiktionary, CC BY-SA 4.0.
codeshare
Today it means: An agreement whereby an airline buys space on another airline and markets the extra space as its own. Often used to increase a route network without the costs of running a full service. Full definition →