A rarely used and rather archaic word that refers to a person with a large foolish grin. A grinagog can also refer to a person who grins all the time, and without sufficient reason.
It was used in the Georgian era and before (during the 1700s) by the lower class, generally in the phrase 'Grinagog, the Cat's Uncle'. No etymology exists, but we can assume that Grinagog comes from the words 'grin' and 'agog'. Saying that someone grins like 'the cat's uncle' is uncommon, but is still in use today. What the cat's uncle has to do with anything, I do not know.
Pronounced GRIN-ah-gog. It is generally used in a derogatory sense.