To underpin is to justify or support the ideas behind something, the way your extensive research underpins your self-published book on ice cream trucks in the United States.
Whenever you substantiate someone’s claims, theories, or stories, you underpin them. A slightly different way to underpin is to form the basis or foundation of something: “A philosophy of empathy and kindness underpins all the interactions at my wonderful summer camp.” This verb can also be used in a more literal way, to mean “support from below,” the way the foundation of a house underpins everything that’s built on top of it.
support with evidence or authority or make more certain or confirm
support from beneath