SEP's history of the deflationary theory of truth does not explain the origins of the term Deflationary, but does point to the first people who "explicitly defended it".
The deflationary theory has been one of the most popular approaches to truth in the twentieth century, having received explicit defense by Frege, Ramsey, Ayer, and Quine, as well as sympathetic treatment from many others. (According to Dummett 1959, the view originates with Frege.) The following passages all contain recognizable versions of the doctrine, though they differ on points of detail.
(Note: none of the passages quoted afterwards use the word "deflationary")
Who first said used phrase "deflationary theory of truth"? Was it Frege?