The OED cites two only usages. The first is from a sermon by William Ames entitled The Saint's Security against Seducing Spirits, which runs "Do we... see some eminent professor... immerd himself in the dung of worldly wisdom", to which the answer is yes.
The second comes from Robert Browning, whom I had always considered strangely prudish. It's in an obscure poem of his called Aristophanes' Apology (1875). Aristophanes says of those whom he satirises
The only drawback to which huge delight [...]
Why, 'tis that, make a muckheap of a man,
There, pillared by your prowess, he remains,
The word comes straight from the French merde, meaning ordure. And the French have an antonym, démerder, which due to their lax morals they are prepared to use in political speeches. During the Nazi occupation of France Charles De Gaulle addressed the French nation by radio in a rousing speech that ended thus:
Françaises, Français, vous avez de la merde jusqu'au cou. Mais moi, qui suis plus grand que vous, je n'en ai que jusqu'aux genoux. Alors, Françaises, Français, démerdez-vous!
Which translates loosely as: Frenchwomen, Frenchmen, you are up to your necks in shit. But I, who am bigger than you, am only up to my knees in it. So, Frenchwomen, Frenchmen, unshit yourselves!