The story
From the similarity to Norway's arrangement, with plus denoting the addition of a customs union outside of EFTA.
Etymology from Wiktionary, CC BY-SA 4.0.
Norway-plus
Today it means: A proposal for a post-Brexit settlement that would have consisted of membership of the European Free Trade Association (EFTA) and the European Economic Area as an EFTA member state, combined with a separate customs union with the European Union. Full definition →