A severed foot is...
A severed foot is the ultimate stocking stuffer.
This joke is a bit macabre, however it is also humorous (to those who are not too repelled by the grisly nature of the joke). The humor lies in the circumvention of the phrase "stocking stuffer". Normally "stocking stuffer" means "a thing that you would want to find in your stocking", but in this joke it means "something that you could find in a stocking".
It is also sort of weird to realize that stockings were made with feet in mind initially, and so this is subverting the Christmas use of a stocking and going back to the original use.