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.