In Shakespeare's time this word had two very different senses, one of which was the ancestor of the modern usage: "unable to restrain sexual appetite". This is what is meant when Thersites rails against "incontinent varlets" (Troilius and Cressida, V.i.94), or Timon advises matrons (he means "married women") to "turn incontinent" (Timon of Athens, IV.i.3). The other sense is found only as an adverb, meaning "immediately, at once". No giggles are needed, therefore, when Desdemona says to Emilia, of Othello:
"He says he will return incontinent" (Othello IV.iii.11). earlier in the same play there is a usage with an - ly ending: Roderigo says to Iago, "I will incontinently drown myself" (I.iii.302). The two senses are cleverly juxtaposed by Rosalind, when she reports to Orlando the way Celia and Oliver fell for each other so quickly: "they have made a pair of stairs to marriage which they will climb incontinent or else be incontinent before marriage," (As You Like It, V.ii.37).
David Crystal is the author, with Ben Crystal, of Shakespeare's Words, published by Penguin