"low land covered wholly or partly by water, a marsh abounding in coarse vegetation," Old English fenn "mud, mire, dirt; fen, marsh, moor," from Proto-Germanic *fanja- "swamp, marsh" (source also of Old Saxon feni, Old Frisian fenne, Middle Dutch venne, Dutch veen, Old High German fenna, German Fenn "marsh," Old Norse fen, Gothic fani "mud"), from PIE *poino-, from root *pen- "swamp" (source also of Gaulish anam "water," Sanskrit pankah "bog, marsh, mud," Old Prussian pannean "swampland"). Italian and Spanish fango, Old French fanc, French fange "mud" are loan-words from Germanic. The native Latin word was limus or lutum.

"a thick substance concreting in liquors; the lees or scum concreted" [Johnson], 1530s, probably from Middle Dutch modder "filth, dregs," from PIE *meu- (see mud).

"mud transfused with saline or other ingredients at mineral springs, into which patients suffering from rheumatism, etc., immerse themselves," 1798, from mud (n.) + bath (n.).

1590s, "destroy the clarity of" (a transferred sense); literal sense ("to bathe in mud") is from c. 1600; perhaps frequentative formation from mud, or from Dutch moddelen "to make (water) muddy," from the same Proto-Germanic source. Sense of "to make muddy" is from 1670s; that of "make confused, bewilder" is recorded by 1680s. Meaning "to bungle" is from 1885. Related: Muddled; muddling.

late 13c., in place names, "abounding in or covered with mud," from mud + -y (2). Meaning "not clear or pure in color" is from 1580s; extended to sounds by 1960s. Big Muddy as a nickname for the Missouri or Mississippi river is attested by 1825. Related: Muddily; muddiness.

"fish which lives or burrows in mud," c. 1500, from mud (n.) + fish (n.).

"piece of rubber behind the wheel of a vehicle to prevent mud from splashing," 1903, from mud (n.) + flap (n.).

also mudhole, "place full of mud," 1760, from mud (n.) + hole (n.).

1680s, "foundation beam of a dam, railroad, house, or other structure," from mud + sill. The word entered U.S. political history in a figurative sense in a speech by James M. Hammond (1807-1864) of South Carolina, March 4, 1858, in the U.S. Senate, alluding to the necessary lowest class of society. In the speech he also took the ground that Northern white laborers were slaves in fact, if not in name. The term subsequently was embraced by Northern workers in the pre-Civil War sectional rivalry.

In all social systems there must be a class to do the menial duties, to perform the drudgery of life. This is a class requiring but a low order of intellect and but little skill. Its requisites are vigor, docility, fidelity. Such a class you must have, or you would not have that other class which leads progress, civilization, and refinement. It constitutes the very mud-sill of society and of political government; and you might as well attempt to build a house in the air as to build either the one or the other except on this mud-sill. [Hammond]