mid-13c., "cow dung and vegetable matter spread as manure," from a Scandinavian source such as Old Norse myki, mykr "cow dung," Danish møg; from Proto-Germanic *muk-, *meuk- "soft." Meaning "unclean matter generally" is from c.1300. Muck-sweat first attested 1690s.

late 14c., "to dig in the ground," also "to remove manure," early 15c., "to spread manure, cover with muck," from muck, n.. Meaning "to make dirty" is from 1832; in the figurative sense, "to make a mess of," it is from 1886; to muck about "mess around" is from 1856. Related: Mucked; mucking.