"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 splash, dabble in water," 1580s, from plash (n.) and also imitative (compare Dutch plassen, German platschen). An earlier form of splash. Related: Plashed; plashing.
"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.