late 14c., "flowing, capable of flowing; neither solid nor gaseous," from Old French liquide "liquid, running" (13c.), from Latin liquidus "fluid, liquid, moist," figuratively "flowing, continuing," also of sounds and voices, from liquere "be fluid," related to liqui "to melt, flow," from PIE *wleik- "to flow, run."
In English, of sounds from 1630s. Financial sense of "capable of being converted to cash" is first recorded 1818, from earlier use in Scots Law (17c.) in reference to debts that had been proved (in court, etc.).
"a liquid substance," 1708, from liquid (adj.). Earlier it meant "sound of a liquid consonant" (1520s), following Latin liquidae, Greek hygra, applied to letters of an easy, "flowing" sound.