1) To kill someone gruesomely.
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.