duller

From dull (verb) +‎ -er.

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: […] Humphrey Robinson, […], and for Humphrey Moseley […], published 1679, →OCLC, Act II, scene ii:

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

• tooth duller

From dull (adjective) +‎ -er.

duller

1. comparative form of dull: more dull