He certainly didn't scoop T.S. Eliot on bullshit, but he might still have been the first poet to use the excretion of bodily wastes as a metaphor for the deprecated expression of ideas. Dryden was definitely the one who invented the idea that preposition-stranding is wrong, but it's odd to think that someone could have invented the Language is Excrement metaphor. This connection seems so natural that I thought it must be as old as the concepts are, like the relation between increasing and rising. (Or is there a culture where your age goes down as you get older?) However, a bit of poking around in the excremental vocabulary of the classical languages failed to discover any examples of crap words used in a figurative sense to describe the expression of false or foolish or shoddy ideas. And a search of LION turned up a poem from the late 17th century, entitled "A Familiar Epistle to Mr. Julian, Secretary to the Muses", and variously attributed to Dryden and to George Villiers, Duke of Buckingham, in which this metaphor is proclaimed:
1 Thou common-shore of this Poetick Town,
2 Where all our Excrements of Wit are thrown:
3 For Sonnet, Satire, Baudry, Blasphemy,
4 Are empty'd and disburden'd all on thee.
5 The cholerick Wight untrussing in a Rage,
6 Finds thee, and leaves his Load upon thy Page.
LION says that "The attribution of this poem is questionable", and does not date its composition, but Villiers lived 1628-1687, and Dryden 1631-1700. Pending new claims (and I'll be surprised not to get some), I'll take this to be earliest documentation of the Language is Excrement meme, of which the term bullshit is a later instance.
Another candidate from the same LION search is an extraordinary piece of poetic vituperation by John Oldham, entitled "Upon the Author of a Play call'd Sodom", and dated 1680. He certainly makes many comparisons between deprecated writing and various noxious materials:
22 Vile Sot! who clapt with Poetry art sick,
23 And void'st Corruption, like a Shanker'd Prick.
24 Like Ulcers, thy impostum'd Addle Brains,
25 Drop out in Matter, which thy Paper stains:
26 Whence nauseous Rhymes, by filthy Births proceed,
27 As Maggots, in some T---rd, ingendring breed.
and he ends with his own excremental comparison, where however the crucial analogy seems to be between a deprecated work and pieces of toilet paper:
Posted by Mark Liberman at August 20, 2005 10:57 PM47 Or (if I may ordain a Fate more fit)
48 For such foul, nasty, Excrements of Wit,
49 May they condemn'd to th'publick Jakes, be lent,
50 For me I'd fear the Piles, in vengeance sent
51 Shou'd I with them prophane my Fundament)
52 There bugger wiping Porters, when they shite,
53 And so thy Book it self, turn Sodomite.