mud: wet earth soft enough to sink into or to knead, from the sludge at the bottom of a pit to the clay a potter treads
This is the plain word for wet earth: the soft, sticky stuff underfoot after rain, on the floor of a dry well, on the bed of a river. Hebrew keeps one word for it whether the mud is a nuisance, a danger, or a raw material, so the same noun turns up in a prayer for rescue, in a battle taunt, and in a brickyard.
Most often it names mud as something that has power over you. A man lowered into an empty cistern does not land on stone, he sinks. Someone praying for rescue asks not to go under. The writers set the word beside other terms for bog and sludge (יָוֵן, רֶפֶשׁ) so that the ground itself feels like a trap.
A second use is a fixed comparison: the mud of the streets. In a walled town the lanes collected the runoff, the refuse, and the traffic of everyone in it, so street mud was the cheapest and filthiest thing a writer could name. To be made like it is to be trampled and counted as nothing, and when a wealthy city heaps up gold "like the mud of the streets" the point is that even gold has become that ordinary. Dry loose earth (עָפָר) usually stands in the same line as its companion image.
The last use points the other way. Wet earth is exactly what a potter or a bricklayer wants, and once it is softened and trodden it becomes workable. In those verses English has to switch words, to "clay" or "mortar", for what Hebrew still calls mud. The regular word for the prepared material itself is חֹמֶר, and it stands right next to this one in both of those passages.