"sacred poem or song," especially one expressing praise and thanksgiving, Old English psealm (West Saxon sealm; Anglian salm), partly from Old French psaume, saume, and 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 and collected in the Old Testament Book of Psalms. Related: Psalmodize. After some hesitation, the pedantic ps- spelling prevailed in English, as it has in many neighboring languages (German, French, etc.), but English is almost alone in not pronouncing the p-.