"causing fear; dreadful, terrible"
Fearful is one of those interesting words where two opposed senses came into the language at about the same time. In addition to its causative sense, the dominant one today, there was also a subjective sense, where the fear comes from within the person ("full of fear"). When Cassius talks of a "fearful night" (Julius Caesar, I.iii.126), the sense is causative. But when Clifford talks of "the fearful French" (Henry VI Part 2, IV.viii.41) he means they are frightened not frightening. And a similar sense of "timid" or "timorous" is found when John describes a messenger as having a "fearful eye" (King John, IV.ii.106) or the Friar calls Romeo a "fearful man" (Romeo and Juliet, III.iii.1). Sometimes only context can decide the meaning, as when Warwick offers York the throne in "the palace of the fearful King" (Henry VI Part 3, I.i.25). At this point Warwick and York are on top: the sense has to be subjective.
David Crystal is author, with Ben Crystal, of Shakespeare's Words, published by Penguin