2. to slit the throat of an unsuspecting victim by a straight razor.
3. to hide the crimes by butchering the corpses of victims, baking their flesh into meat pies, and selling them to unknowing customers.
2) Originates from the movie/musical known as 'Sweeney Todd'. In this movie, Johnny Depp plays the role of Sweeney Todd, a barber that slits peoples throat while giving them a shave.
3) To Sweeney todd someone, you would have to kill them in the same manner as Sweeney Todd himself of an alternative way that is just as horrific.