mid-15c., "moderately well," 1520s, "indifferently, neither too poorly nor too well," from so (adv.), which is attested from mid-13c. in the sense "in this state or condition." As an adjective, "mediocre, neither too good nor too bad," 1540s. So-and-so is from 1596 meaning "something unspecified;" first recorded 1897 as a euphemistic term of abuse. In 17c.-18c. So, so also could be colloquially a mere introductory phrase.