fundament

noun
  1. The foundation or basis of something; the underlying principle or concept. noun
    Understanding the fundament of mathematics is crucial for advanced studies.
    The fundament of her argument was based on solid evidence.
  2. The buttocks or the part of the body on which one sits. noun
    He landed heavily on his fundament after slipping on the ice.
    The chair was uncomfortable, causing pain in his fundament.