I'll have to check on my old books to confirm, for I'm giving you this from memory, but you certainly have something to start from here. There are a lot of idioms including 'kuti'. I cannot list them all. In any case, the metaphor you refer to seems to be part of a larger digestive-tract metaphor for language and speech. Apparently mental analysis and evaluation are equivalent to mastication.
Suspicions are smells.
Mysteries are hard nuts to crack, while ideas that don't have a leg to stand on are a bunch of crap, bullshit, or other waste extensions. So material texture and toughness are involved here too as bases for ideas of complexity of form and substance. Differences in how language is perceived may be reflected in what upper digestive tract body part is chosen to represent it. I have data for you from Setswana. A language spoken mainly in Botswana and South Africa, and certain small parts of Namibia and Zimbabwe.