Hey, whatever it takes to move product, right?
"Lost in the mists of time," according to Wikipedia. That's Wikipedo for "we don't know, we don't know how to find out, we're too lazy to find out, so quit asking."
Has something to do with some brand of grill called Cleveland Steamers, which are steam-based cooking machines. While this may be amusing for some it's irrelevant, but it was brought into the Wikipedia discussion to provide yet another "pop culture reference," which TOW thrives on.
It may actually be as simple as what the original alt.sex poster said: Cleveland smells dirty, and turds steam.
Perhaps the obvious sockpuppet "Lord Pedicabo," writing on the Cleveland Steamer talk page was right. At one time, a few decades ago, Lake Erie, Cleveland's waterfront, was a dead lake because of industrial waste. At the same time, people would routinely dig clams on beaches to steam for eating. These were called "steamer clams," and were an essential part of a clambake. So, perhaps the two concepts were conflated, and a "Cleveland steamer" became a turd.
This is complete speculation, but the "Cleveland steamer" may have originated as a joke played on people in the context of a bar. A man may be drinking a gin and tonic, a respectable drink, and he may be accosted by a bar regular who asks him if he'd like to try something stronger and more manly, like a Cleveland steamer. Hilarity then ensues, especially when it is revealed that a truly fugly waitress will be serving his steamer.