Plimsoll is the full name, but of course there were nicknames for them. According to wiki they wee indeed sand shoes originally!
https://en.wikipedia.org/wiki/Plimsoll_shoe
A plimsoll shoe, plimsoll, plimsole, daps or pumps (British English; see other names below) is a type of athletic shoe with a canvas upper and rubber sole developed as beachwear in the 1830s by the Liverpool Rubber Company.
Plimsolls have solid rubber soles about 8 or 9 mm thick, to which the canvas is glued without coming up the sides (as on trainers). The effect when running is similar to running without shoes.
The shoe was originally, and often still is in parts of the United Kingdom, called a "sand shoe" and acquired the nickname "plimsoll" in the 1870s. This name arose, according to Nicholette Jones's book The Plimsoll Sensation, because the coloured horizontal band joining the upper to the sole resembled the Plimsoll line on a ship's hull, or because, just like the Plimsoll line on a ship, if water got above the line of the rubber sole, the wearer would get wet.[1]
