Most of the meanings of want found in Shakespeare are still in use today; but there is an inevitable tendency to read in the primary modern meaning - the positive sense of "desire" - in contexts where it does not work. It is the negative sense, of "lack, be without" which is required when Cordelia says to Lear, "I want that glib and oily art To speak and purpose not" (King Lear, I.i.225). This could not possibly mean that Cordelia desires to be glib: she is distancing herself from her two sisters, whom she has just heard speaking in that way. Similarly, in the Epilogue to The Tempest (line 14), when Prospero says "Now I want Spirits to enforce, art to enchant" he does not mean that he desires spirits, for he has just sent them all away. He is reflecting on their absence. Over half of Shakespeare's uses of want are like this.
David Crystal is author, with Ben Crystal, of Shakespeare's Words, published by Penguin