late 14c., "plug used to stop the hole of a cask," according to Barnhart probably from Old French *espigot (compare Gascony dialect espigot "core of a fruit, small ear of grain"), diminutive of Old Provençal espiga "ear of grain," from Latin spica "ear of grain" (see spike (n.2)). Meaning "valve for controlling the flow of a liquid" is from 1520s; the connecting notion is "that which controls or restrains."
