If I say "They were in cahoots", 'cahoots' makes most sense as a noun.
There are different kinds of nouns. I'm sure different linguistic systems divide them up differently. For instance, there are nouns that refer to objects (trees, rocks, people), nouns that refer to locations, to categories, to states of being (confusion, death) or to abstract concepts.
What noun category do people reckon 'cahoots' falls into?
Of course, you could probably mount an argument that 'in cahoots' is actually a kind of adjective. After all, if we say "He is in despair", we wouldn't consider 'despair' to be a noun, like a location.