1. comparative form of dull

duller (plural dullers)

1. One who, or that which, dulls.
   • c. 1608–1610, Francis Beaumont; John Fletcher, “Philaster: Or, Love Lies a Bleeding”, in Comedies and Tragedies […], London: Printed for Humphrey Robinson, […], and for Humphrey Moseley […], published 1679, OCLC 3083972 ↗, Act 2, scene 2:

     But, of all, your grace must fly phlebotomy, fresh pork, conger, and clarified whey: they are all dullers of the vital spirits.