"It was my word against Hagrid's, Harry. Well, you can imagine how it looked to old Armando Dippet. On the one hand, Tom Riddle, poor but brilliant, parentless but so brave, school prefect, model student ... on the other hand, big, blundering Hagrid, in trouble every other week, trying to raise werewolf cubs under his bed, sneaking off to the Forbidden Forest to wrestle trolls ... but I admit, even I was surprised how well the plan worked."
-- Tom Riddle, via the Diary (CS17)
Armando Dippet is a former headmaster of Hogwarts (c. 1942 through mid-1960s), Dumbledore’s predecessor, who was totally taken in by Tom Riddle. Dippet’s portrait hangs in Dumbledore’s office (OP37). In life, he was nearly bald and somewhat feeble (CS13).
Tom Riddle, when he had finished school, asked Dippet if he could stay on as a teacher, but Dippet told him that at eighteen he was too young (HBP20). During Dippet’s tenure as Head, an infamous performance of a pantomime of The Fountain of Fair Fortune nearly burned down the Great Hall (TBB/FFF).
Rita Skeeter wrote a tell-all biography of Dippet entitled Armando Dippet: Master or Moron? (DH13).
"dip" = U.S. slang for someone who is rather inept and silly.