The origins of the surname Dennis have been described in various published histories of surnames. Dionysus, the Greek god of wine, was also the first name of the first Bishop of Paris, in the 3rd century AD. Once he was sanctified, and the French city of Saint Denis was named after him, the name became popular as a first name in France. A town near St Austell in Cornwall also bears the name St Dennis.
Or it could come from Danais, the old French word for Danish. The Norman name was originally Le Danais. Sir Henry Le Danais was the administrator of the Pomeroys, the right hand man of William the Conqueror. Members of this family are still highly placed in the French Civil Service. Some Le Danais family members came to England with the Norman invasion. One of the earliest records of the name in England was in the county of Lancashire.
The name Dennis is also found in Ireland, and in a twist on the Irish joke routine, a 'wag' on the internet suggests the original Irish Dennis was thrown out of the country because he sinned and had to change his name - backwards! Jokes aside, a Dennis who is famous in Australia, the poet C J Dennis, is a descendant of an Irish Dennis family.
Although not listed as a Cornish surname in White's book*, Cornish historians such as CraigWeatherhill recognise the surname Dennis as distinctively Cornish, derived from the prehistoric word 'dynas' meaning fort, and originating in the St Colomb Major area between Newquay on the Atlantic coast and St Austell on the English Channel. The word 'Pendennis' in the famous Pendennis Castle site at Falmouth means 'fort on the headland'.
* G. Pawley White, A Handbook of Cornish Surnames (G.P. White, Camborne, 1972)