Dai means big, as in the Daibutsu. Con is short for “convention,” as in science fiction conventions.
That’s why the gentleman in the photo has a giant radish on the back of his tunic.
In 1983, at Daicon 4, a group of animators premiered a fan video. The over the top sequence introduced several visual tropes identified with the team who’d later become Studio Gainax (Evangelion, Kare Kano, FLCL.)
