Where does “hrom” come from?

hrom (Old Czech) comes from Proto-Slavic gromъ, from Proto-Indo-European gʰrom-o-, from Proto-Indo-European gʰrem-, from Proto-Indo-European gʰer- — to rub; to stroke; to grind.

Ancestry of “hrom”, step by step

Step	Language	Word	Meaning
1	Proto-Slavic	gromъ	thunder
2	Proto-Indo-European	gʰrom-o-	—
3	Proto-Indo-European	gʰrem-	to be angry; to sound, bellow, growl, roar, be...
4	Proto-Indo-European	gʰer-	to rub; to stroke; to grind

Every word from Proto-Indo-European gʰer- →

hrom — Etymology, Origin & Word History | Etymology Explorer