What does the D stand for in "D Flip-Flop"?
Montgomery Phister's textbook "Logical design of digital computers" from 1958 (written in 1957) introduces the D flip-flop as "the delay memory element".



This doesn't prove anything but it suggests that already in 1957, at least, there was tradition to suggest that the D means "delay".

As a side-note to put things into historical perspective, here's a schematic for a flip-flop from the same book, using radio tubes and 200 volt anode voltage.