An Etymological Dictionary of the German Language/Hagel
Hagel, m., from the equiv. MidHG. hagel, OHG. hagal, m., ‘hail’; comp. Du. hagel, m., AS. hagol, hœgel, m., E. hail; OIc. hagl, n.; the common Teut. word for ‘hail,’ by chance not recorded in Goth. only. A single pebble was called a ‘stone.’ OIc. haglsteinn, AS. hœgelstân, E. hailstone, MidHG. and earlier ModHG. Hagelstein. Comp. ModHG. kieseln, ‘to hail,’ Kieselstein, ‘hailstone.’ Perhaps Hagel itself signified orig. nothing but a ‘pebble’; at least there are no phonetic difficulties against the derivation from pre-Teut. kaghlo-, ‘flint-stone’ (comp. Gr. κάχληξ, ‘small stone, pebble’).