DULL is validvalid

mentally slow [adj DULLER, DULLEST] : DULLNESS [n], DULLY [adv] / to make less sharp [v DULLED, DULLING, DULLS]

2,149 of 196,601 in NWL2023·also valid in CSW24

mentally slow [adj DULLER, DULLEST, DULLY] / to blunt, make dull [v -ED, -ING, -S]