"small puddle, shallow pool, wet ground," Old English plæsc "pool of water, puddle," probably imitative (compare plash (v.1) and Dutch plass "pool"). Meaning "noise made by splashing" is recorded by 1510s. Related: Plashy.
"to interlace, to bend and interweave the branches or twigs of," late 15c. (implied in plashing), from Old French plaissier, from Latin plectere "to plait," from suffixed form of PIE root *plek- "to plait." Related: Plashed.