Comment by vidarh

It's to an extent just a saying, probably based on it often feeling like that, in that the physical sensations of some feelings are linked to parts of the body.

But specifically with respect to the gut, the gut has a huge number of nerve cells that act like reward neurons, can directly trigger changes to hormone levels, and has a very substantial direct connection to the brain (the vagus nerve), so it's reasonable to say that we do genuinely have "gut feelings".

https://www.science.org/content/article/your-gut-directly-co...