[adv] with fatal consequences or implications; "he was fatally ill equipped for the climb"
FATALLY is a 7 letter word that starts with F.
Webster's 1913 Dictionary
Definition:
\Fa"tal*ly\, adv.
1. In a manner proceeding from, or determined by, fate.
--Bentley.
2. In a manner issuing in death or ruin; mortally;
destructively; as, fatally deceived or wounded.
WordNet Dictionary