Soundless comes from sound (noise) + -less. Sound derives from Old French son, soun, from Latin sonus ('sound'), from Proto-Indo-European *swen- ('to sound, resound'). The historical chain includes Latin sonus and Proto-Indo-European *swen-. The quoted older senses, "sound" and "to sound, resound", supply the semantic anchors for soundless. The formation is best read as suffixal and borrowed: the word soundless carries its older form-history into a compact English lexical shape. In present English it is used as a adj, "making or accompanied by no sound; silent". English derivatives include soundlessly and soundlessness; related English forms include sound. The result is a adj whose etymology preserves both the older form and the later English specialization.