means A broad, level stretch of muddy ground exposed at low tide along coasts, estuaries, and tidal rivers.

from A plain compound of "mud" and "flat," both old workhorse English words. "Mud" is medieval, likely borrowed from Low German or Middle Dutch (compare modder), and "flat" comes from Old Norse flatr, meaning level — a word the Vikings left behind like a stone in the silt. Together they describe exactly what they are: flat mud, named by people who walked it.