Where does the pet name Unca Cece for Cecil come from?
I think Cecil has used it himself is some of his columns.
This seems to go back to the early days of the column. The initial Straight Dope book was first published in 1984. In the answer to the very first question appearing in this book, Cecil refers to himself as “Uncle Cecil”. On page 65, he writes “Take it from your Unca Cecil”. There are also references to “Cece” by various questioners.
I’d guess that the “Uncle/Unca” references originated with Cecil, while “Cece” came from the Teeming Millions. Eventually, the two were combined.