The sound comes from Old MacDonald Duck, a Donald Duck cartoon released in 1941, in which Donald runs a farm and milks a cow. The clip itself is catalogued as a Hollywood Edge water squirt effect, which explains its clean, close-up splash. Short cartoon effects like this later spread through soundboard sites and meme edits, where a single recognisable noise can carry a joke on its own. It remains useful because the rhythm is instantly readable as milking, even in a clip only a fraction of a second long.