Fearful means full of fear, or causing it. Fearful of what might happen, a fearful noise, fearful consequences.
The two senses point in opposite directions — a fearful person is afraid and a fearful noise causes fear — and only context separates them, which is unusual and occasionally genuinely ambiguous. Fearful of is the construction for the first. In British informal use, fearful is also an intensifier meaning very bad: a fearful mess. Note fearsome, which only means causing fear; and fearless, which only means without it.
1adjectivefearfulFull of fear.
“The villagers were fearful of another flood.”
2adjectivecausingCausing fear, or very bad.
“There was a fearful noise at roughly four in the morning.”
fearful is a quality of fear.
The word is fear plus -ful, and -ful once meant full of rather than causing. English let the ending drift both ways, which is why fearful faces two directions and hopeful faces only one.
Middle English
ferful
English, today
fearful
From Middle English ferful, fervol, equivalent to fear + -ful.
Etymology from Wiktionary, CC BY-SA