At the commencement of the sales, he gave every one that wanted to purchase a paper containing a description of the lands that were to be sold; and, as the sales were cried, he called over the numbers and described the land; and when it got up to one dollar and a quarter an acre, if no body bid, after it was cried two or three times, he would say, knock it off, knock it off. [U.S. Senate record, 1834]
The origin could possibly be from the habit of knocking a special beat to indicate a change of oarsmen in slave galleys. That's speculation but several of the earliest references to the term come from a nautical source; for example, William Clark Russell's novel An ocean tragedy, 1890:
"We were forced to knock off through sheer fatigue."