1530s, "the fall of a heavy body or blow," probably imitative. It also meant "pig-wash, filth, wet refuse" (1520s) and may have been imitative of the sound of water dashing against solid objects. The meaning "a body of splashing water" is first found 1670s; that of "a dashing or splashing" is from 1847. Swash-letters (1883) are italic capitals with flourished projections.
1580s, "spill or splash (water) about," 1530s, possibly from wash (v.) with an intensifying s-, or imitative of the sound of water dashing against solid objects. Related: Swashed; swashing.