Fearful means “terrifying, awful,” and it also means “afraid, full of fear” and “timorous.” We saw a fearful animal is ambiguous, since you can’t tell from the sentence whether the animal fears us or causes us to fear it. Making fearful even more elusive is its latest and increasingly frequent use to mean “huge, great,” usually applied to things thought unpleasant or bad, as in a fearful crash or a fearful cold.
Fearsome means “dreadful, frightening, causing fear” but also has an opposite sense, “frightened, timorous,” which causes confusion just as do the contradictory senses of fearful. Good advice: when you use either fearful or fearsome to mean “frightened,” state the cause of the fear, too (She’s fearful [fearsome] of the dark). Better still: don’t use fearsome in that sense at all; use it for the horrible, not the timorous.
Fearfully, the adverb, has meanings similar to the adjective fearful, but it has gone on to develop into an intensifier as well: I have a fearfully bad cold. Fearsomely is an adverb meaning “horribly, frighteningly,” and it seems not to have developed beyond nonce word use as an intensifier.
