The only free and comprehensive online etymological dictionary of the Spanish language
grave (Adjective) "heavy," "serious"
10th cent. Old Spanish grave "heavy," "serious," "difficult." The meaning of "difficult" was eventually replaced in Old Spanish by difícil. From Latin gravis 'id.' From Proto-Italic *grawus 'id.' From Proto-Indo-European *gwreh2-u̯s 'id.' From the root *gwreh2- "to be heavy."
Indo-European
Romance
Portuguese grave, Catalan greu, French grave, Italian grave, Aromanian greu, Romanian grev
Italic
Oscan bravús
Germanic
Gothic *kaurus
Hellenic
Hellenic Ancient Greek βαρύς (barys) "heavy" (whence bajo)
Indo-Iranian
Sanskrit gurú-
Tocharian
A krāmärts, B krāmär "weight"