The plimsoll symbol ⦵ (or o) that is used as a superscript in the notation of thermodynamics to indicate an arbitrarily chosen non-zero reference point.
A rubber-soled lace-up canvas shoe for sports or onboard ships; a precursor of trainers.
How often have the words football boot and plimsoll occurred in a corpus of books? (source: Google Ngram Viewer )