The splash knock comes from The Plastics Inventor, a Donald Duck cartoon released on September 1, 1944. In the short, Donald's homemade airplane antics are scored with exaggerated cartoon effects, including the familiar shave and a haircut rhythm capped by watery splashes. The shave and a haircut riff itself is a much older vaudeville and music-hall gag that became a universal comedy cue. Clipped for soundboards, the combination survives as a quick shorthand for a cheeky knock followed by a wet, slapstick payoff.