There are 14 12-letter words that start with 'eff'.
| effectuality | effectuating | effectuation | 
| effeminacies | effervescent | effervescing | 
| effetenesses | efficacities | efficiencies | 
| efflorescent | efflorescing | effortlessly | 
| effronteries | effusiveness | |
Note: These 14 12-letter words that start with 'eff' are found from the USA & Canada version of the tournament Scrabble word list.
Also see words start with effe, effl, effu, effi, effa, effo, effr, effs.
4-letter words start with 'eff' (1)