Solidus.

Mark Liberman’s Log post inspired me to discuss the very interesting word solidus ‘slash (mark).’ It developed from the name of a gold coin of the Roman Empire, which gave rise to the Latin name for a variety of medieval coins, mostly made of silver: Byzantine, Carolingian, Italian, and — the one which concerns us here — the English shilling. The word shilling was, understandably, shortened to its first letter, for which they used the “long s”: ſ. This was frequently written as a simple slash mark, so, to quote the OED s.v. solidus (sense 2): A sloping line used to separate shillings from pence, as 12/6, in writing fractions, and for other separations of figures and letters; a shilling-mark. (Their first citation is “1891 in Century Dictionary, which can presumably be antedated — the entry is from 1913.) As Wiktionary tells us, it is a doublet of sol, sold, soldo, solid, sou, and xu (a hundredth of a dong).