It’s not the first definition, or even the second, but if you scroll down to the third definition of the word fundament, OED will give you:
The lower part of the body, on which one sits; the buttocks
With one of many examples being:
1754 Connoisseur No. 5. ⁋12 Applying his foot directly to my fundament.
This is a grand old word, dating back to the 1200s, and coming pretty much as is from Latin. I like it, and probably use it at least least once in every book. Why? Because I’m dealing with lords and ladies who are both reasonably well educated (the gentlemen probably qualifying as fluent in Latin), and yet–because they populate romance novels–these are also people with a healthy fascination with certain aspects of the opposite sex. Finding ways for educated, if lusty, Regency people to talk about that fascination can become a challenge to my vocabulary–a fundamental challenge.