fundament.
“Fundament” = (1) basis; or (2) anus or buttocks. Sense 2 is more common in British English than in American — e.g.: “There is even a 12-step group for people addicted to 12-step groups — which is very Fight Club, but surely only a small step away from one’s head disappearing entirely up one’s own fundament.” Mimi Spencer, “Let’s Talk About Me,” Sunday Times (London), 19 Jan. 2003, Features §, at 12.
But because sense 2 is current in American English as well, it typically can’t be used without creating a double entendre. One hardly knows what to think about sentences such as this: “This is the best of the best, and like it or not, it’s the fundament of our private culture and public life.” Kyrie O’Connor, “‘Great Books,’ with a Classic Male Slant,” Hartford Courant, 22 Sept. 1996, at G3. To avoid trouble, try “foundation.”
Sometimes the word is misused in the plural as an inelegant variation of “fundamentals” — e.g.: “[Kyle] Korver is like a golfer, always tinkering, always looking for the perfect swing. He’ll watch other players, work on his fundaments [read ‘fundamentals’], tweak his own shot.” Tom Shatel, “Korver Puts in the Hours to Become Jays’ Big Shot,” Omaha World-Herald, 18 Jan. 2003, at C1. (He wouldn’t get far by working on his fundament.)
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Quotation of the Day: “What is biography? Unadorned romance. What is romance? Adorned biography. Adorn it less & it will be better than it is.” Mark Twain (as quoted in 3 Mark Twain’s Notebooks & Journals 239 (Robert Pack Browning et al. eds., 1979)) (as quoted in Mark Twain: His Words, Wit, and Wisdom 26 (R. Kent Rasmussen ed., 1997)).