The modern senses were well established in Shakespeare's time, but several other uses were entering the language in the 1600s which have not lasted into Modern English. The commonest in Shakespeare is "suspicious, wary, watchful", as when Cassius tells Brutus "be not jealous on me" (Julius Caesar, I.ii.71) or Edmund describes Gonerill and Regan as being "each jealous of the other" (King Lear, V.i.56). In this sense it is not restricted to a human emotion: daytime, for instance, can be "jealous" (in The Rape of Lucrece, l.800). When Olivia refers to her soul as "jealous", she means it is "anxious, worried" (Twelfth Night, IV.iii.27), and this is the sense required when Erpingham tells Henry that his nobles are "jealous of your absence" (Henry V, IV.i.278). It means "solicitous, zealous" when Jaques talks of a soldier being "jealous in honour" (As You Like It, II.vii.152). And it means "doubtful, uncertain" when Brutus tells Cassius "That you do love me, I am nothing jealous" (Julius Caesar, I.ii.161) - a first recorded usage in English.
David Crystal is author, with Ben Crystal, of Shakespeare's Words, published by Penguin