A footnote in my first year university chemistry book (General Chemistry by Linus Pauling) states: "This use of the letters s, p, d, f has a curious origin; they are the initial letters of the adjectives sharp, principal, diffuse and fundamental, which happened to be used by spectroscopists in the period around 1890 to describe different observed spectral series of the alkali metals. These letters accordingly are not abbreviations of the words that describe the orbitals in a meaningful way."