When this word came into English, in the 15th century, it was immediately used in two diametrically opposed senses: "feeling fear" and "causing fear". Only the former sense is found today. Shakespeare uses the word half a dozen times, usually in the same way as we do now, as when the French General tells Talbot "The Dauphin's drum ... Sings heavy music to thy timorous soul" (Henry VI, Part 1, IV.ii.40), or, six lines later, Talbot talks of his army as "A little herd of England's timorous deer". But the word can hardly mean "fearful" when Iago tells Roderigo to raise the alarm by calling aloud '"with like timorous accent and dire yell, As when, by night and negligence, the fire Is spied in populous cities" (Othello, I.i.76). It takes something other than a fearful voice to warn everyone about a fire.
David Crystal is author, with Ben Crystal, of Shakespeare's Words published by Penguin.