the alcoholic's version of the holiday known as Cinco de Mayo, a holiday held on May 5 that commemorates the Mexican army's unlikely victory over French forces at the Battle of Puebla on May 5, 1862.
A 'dincc' is someone who has missed something completely obvious or someone who has been incorrect in something very easy. It could also easily be a simple misunderstanding For example, if someone were to accidentally (or on purpose) that Africa was a country or 7*5=45. It is a mild insult often followed by laughter.