@Maroon got it right in her comment.

Ruminoir is listed in no dictionary but immediately evoques the verb ruminer to native ears.

Ruminer means to ruminate and can also be used figuratively to mean "to dwell on something", i.e. instead of bringing back food to chew it like cows of sheep, bringing back thoughts in the mind again and again.

Despite having no "legal" existence, ruminoir appears sporadically and means "the place where rumination occurs".

In your quote, the sense is literal: its rumen, its gullet.

A figurative meaning is more often observed, either a place where someone is alone and dwell on things, as @Laurent S. suggested, it is close to boudoir but pejorative, or, more rarely, a period of time when someone didn’t act a lot, a traversée du désert (a barren spell).