Who was the original person whose skull is used by Hamlet when he held it up and announced "Alas, poor Yorick"?

There is no deception in this proclamation. Yorick was in fact a jester to Hamlet’s father when Hamlet was a boy. “A man of infinite jest and most excellent fancy” he would pick up Hamlet and give him rides on his back (“He hath borne me on his back a thousand times.”) So this figure from Hamlet's childhood is now long-dead, and is now deteriorating in his grave. (A common practice in these times is to disturb one grave to dig another.) The whole scene is re-establishing one of the themes of the play, the dichotomy between the physical world of flesh and bone, and the possibility of a “spiritual” world after death. It is also an excellent example of how Shakespeare uses Horatio as a character whose function in the dramaturgy is to be a sounding board that allows us, the audience, to be privy to Hamlet's thoughts.