"One theory holds that the sign grew out of the Spanish and Spanish American scribal abbreviation "ps" for pesos. A study of late 18th- and early 19th-century manuscripts shows that the s gradually came to be written over the p, developing into a close equivalent to the "$" mark."

https://en.wikipedia.org/wiki/Dollar_sign#Use_for_the_Spanish_American_peso_in_the_late_1700s