Old English psealm, salm, partly from Old French psaume, saume, partly from Church Latin psalmus, from Greek psalmos "song sung to a harp," originally "performance on stringed instrument; a plucking of the harp" (compare psaltes "harper"), from psallein "play on a stringed instrument, pull, twitch" (see feel (v.)).
Used in Septuagint for Hebrew mizmor "song," especially the sort sung by David to the harp. Related: Psalmodize; psalmody. After some hesitation, the pedantic ps- spelling prevailed in English, as it was in many neighboring languages (German, French, etc.), but English is almost alone in not pronouncing the p-.