"instrument for bringing up or removing solid matter from under water by dragging the bottom," late 15c., in Scottish dreg-boat "boat for dredging," perhaps ultimately from root of drag (possibly via Middle Dutch dregghe "drag-net").

"to clear out with a dredge, gather with a dredge," also figurative, c. 1500 in Scottish, from dredge (n.). Related: Dredged; dredging. 

c. 1500, "one who works or makes use of a dredge," agent noun from dredge (v.). As "boat employed in dredging," from c. 1600.