The OED’s suggested etymology links the word’s second half -sion with Old English shench ('a cupful, drink (of liquor)’) and its related verb scencean (‘to pour a liquid’). As a result the word is usually interpreted as a ‘drink taken in the afternoon’. The fact that in these Anglo-Norman attestations the nonsion seems to be listed as part payment included in the working days of carpenters raises the possibility that it might have been a more substantial snack or even small meal eaten at work.
What is clearly an English or Old English word first emerges in Latin documents as a ‘foreign’ vernacular loanword in the thirteenth century. We then see it appearing in mid-fourteenth-century Anglo-Norman texts, where it is used as a ‘normal’ French word, and a few decades later in Latin in the same way. It is only half a century later (around 1422-23) that we have the first surviving fully English documents that use the word.
The English word luncheon (OED luncheon n.), which is similar in form (and perhaps etymology?), is considered post-medieval.