Die-dum

A die-dum, according to Gregory Goyle, is what Harry was looking for in the Room of Requirement.

"We was hiding in the corridor outside," grunted Goyle. "We can do Diss-ludion Charms now! And then," his face split into a gormless grin, "you turned up right in front of us and said you was looking for a die-dum! What's a die-dum?"