underpinning

1. (figurative) A basis for something.Examples: "I would like to know more about the underpinning of this political theory."; "Other eras have had their epicureans or hedonists, of course, who lived and sometimes died for the ideal of simplicity or pleasure in an epic kind of way. But today’s gazillion-strong influencer community has zero intellectual underpinning to it, which is why it’s so often the unwitting butt of its own joke."figuratively

2. (figurative, humorous) The lower body or legs.Examples: "Thinks I to myself, “Sol, you're run off your course again. This is a rich man's summer ‘cottage’ and if you don't look out there's likely to be some nice, lively dog taking an interest in your underpinning.”"figurativelyhumorous