Old English forst, frost "frost, a freezing, frozen precipitation, extreme cold," from Proto-Germanic *frustaz "frost" (source also of Old Saxon, Old Frisian, Old High German frost, Middle Dutch and Dutch vorst), from the verb *freusanan "to freeze" (source of Old English freosan "to freeze"), from suffixed form of PIE *preus- "to freeze; burn" (see freeze (v.)). Both forms of the word were common in English till late 15c.; the triumph of frost may be due to its similarity to the forms in other Germanic languages. A black frost (late 14c.) is one which kills plants (turns them black) but is not accompanied by visible frozen dew.