This here is a tool that looks up the frequency of words used in the New York Times, both in the percentage of and how many total articles the word appeared in. We put it to terrible use.
Above is every instance of "fart," "poop," "dong," and "butt" to have appeared in the New York Times since 1851. Below is the percentage-based chart. As you can see, there are some obvious (totally random) correlations. Farts and butts, for instance, moved nearly in lockstep from 1875 to 1877. And since the 1970s, dongs and butts have been tightly correlated.
Sure, not all of the references are strictly biological. Many of the early poop references are nautical, like this, from 1862:
"British steamship Bermuda, two masts, brig-rigged, commander TESSIER; lower part of funnel black, upper portion red; no poop, wheel-house white; carries six white boats and four guns; laden with arms and ammunition."
The butts, often, are gun butts or butts of jokes, or people named Butt, like Charles C. Butt, the chairman and chief executive of the grocery chain H-E-B, which we enjoy.
You can click through each year in the tool to go to an archive of every article containing the given word. Give it a try and put what you turn up in the comments.