There are 15 words that start with 'dull'.
dull | dulls | dully |
dulled | duller | dullard |
dullest | dulling | dullish |
dullards | dullness | dullishly |
dullnesses | dullsville | dullsvilles |
Note: These 15 words that start with 'dull' are found from the USA & Canada version of the tournament Scrabble word list.