The symbol ‘c’ for the speed of light in vacuum originated from a paper by Weber and Kohlrausch in 1856, where they measured a quantity denoted by ‘c’ in an electrodynamics force law equation. This ‘c’ was later referred to as Weber’s constant, equal to the speed of light times the square root of two. The Latin word “celeritas,” meaning “speed,” likely influenced the use of ‘c’, though its original intention was to represent “constant.” Historically, the symbol has roots in the 17th and 18th centuries with various meanings of speed, including the wave equation. Notably, Isaac Asimov’s claim that ‘c’ stands for celeritas has gained traction.
https://math.ucr.edu/home/baez/physics/Relativity/SpeedOfLight/c.html