The modern sense was coming into the language in Shakespeare's time - "This is the silliest stuff that ever I heard," says Hippolyta to Theseus (Midsummer Night's Dream, V.i.207) - but an older set of senses dominate in the plays. When Henry VI's Queen calls herself a "silly woman" (Henry VI, Part Three, I.i.243), or Lodowick describes the Countess in the same way (Edward III, II.i.18), the word means "helpless, defenceless". Males can be silly too - Edward III describes a group of Frenchmen as "poor silly men, much wronged" (IV.ii.29) - and so can sheep and lambs: "shepherds looking on their silly sheep" says Henry VI a little later in Henry VI, Part Three (II.v.43). But when a captain describes the disguised Posthumus as being "in a silly habit" (Cymbeline, V.iii.86), a different sense emerges, of "lowly, humble". We can sense the modern meaning waiting in the wings.
David Crystal is the author, with Ben Crystal, of Shakespeare's Words, published by Penguin