noun. A place where people are held before being killed.

The violent jerks from excess to excess of the patients at Dr Sacks’s pseudonymous New York hospital — ‘not a sanatorium but a thanatorium’, as one of the inmates remarked.
The Times (1976)

The first part of this word is actually from Thanatos — Greek for death. Despite being classified as a nonce-word in the OED, it does have more than one citation. It had a life, albeit short lived and in the same newspaper.

It does, however, seem like it could be useful in certain contexts: in describing the holding place of convicts on death row, an area with livestock ready to be slaughtered, the house of your in-laws, etc.

If you say Sanatorium with a lisp, beware. You might be sending someone to the wrong place.