"small puddle, shallow pool, wet ground," Old English plæsc "pool of water, puddle," probably imitative (compare Dutch plass "pool"). Meaning "noise made by splashing" is first recorded 1510s.
"to splash," 1580s, from plash (n.) and also imitative (compare Dutch plassen, German platschen). Related: Plashed; plashing.
"to interlace," late 15c., from Old French plaissier, from Latin plectere "to plait," from suffixed form of PIE root *plek- "to plait." Related: Plashed; plashing.