An Etymological Dictionary of the German Language/Annotated/Hagel
Hagel, masculine, from the equivalent Middle High German hagel, Old High German hagal, masculine, ‘hail’; compare Dutch hagel, masculine, Anglo-Saxon hagol, hœgel, masculine, English hail; Old Icelandic hagl, neuter; the common Teutonic word for ‘hail,’ by chance not recorded in Gothic only. A single pebble was called a ‘stone.’ Old Icelandic haglsteinn, Anglo-Saxon hœgelstân, English hailstone, Middle High German and earlier Modern High German Hagelstein. Compare Modern High German kieseln, ‘to hail,’ Kieselstein, ‘hailstone.’ Perhaps Hagel itself signified originally nothing but a ‘pebble’; at least there are no phonetic difficulties against the derivation from pre-Teutonic kaghlo-, ‘flint-stone’ (compare Greek κάχληξ, ‘small stone, pebble’).