Borrowed from German Firn, from Alemannic German firn (“last year's”), from Old High German firni (“old”), from Proto-Germanic *firnijaz, *fernaz (“foregoing, previous”), from Proto-Indo-European *per- (“forth, forward, across, through”); cognate with Old English fyrn (“former”), Old Norse forn (“old”), and Gothic ???????? (fairneis, “old”).
firn (countable and uncountable, plural firns)
- A type of old snow which has gone through multiple thaw and refreeze cycles and thus is made of numerous small icy grains, though it is not nearly as saturated with water as snow-cone slush is; can be hard or somewhat soft depending on recent and current weather conditions.