Historical clue formulations

• Dunderheaded — 2025-12-20
• Like tomes, typically — 2021-01-13
• With 66-Across, slow learner in the litter? — 2015-07-01
• Like unabridged dictionaries — 2013-06-15
• Blockheaded — 2012-01-17
• Dense — 2011-05-16
• Not too brainy — 1999-05-04