The underlying theme today is that someone isn't telling the truth or behaving sincerely, and in this sense it can be traced back to the 16th century. The word does sometimes have this sense in Shakespeare, as when Queen Katherine tells Henry VIII off for his taxation policy: "the pretence for this Is named your wars in France" (Henry VIII, I.ii.59). But usually the word lacks any notion of hypocrisy. When one of the lords in All's Well that Ends Well (IV.iii.47) says of Helena's journey, "Her pretence is a pilgrimage", he means only that this is her intention. And it is this sense of "plan" or "purpose" which is the usual one in the plays, as when Edmund tells his father that Edgar has written a letter "to feel my affection to your honour and to no other pretence of danger" (King Lear, I.ii.88).
David Crystal is author, with Ben Crystal, of Shakespeare's Words, published by Penguin