Dutch word grondig comes from Dutch -ig, Dutch grond (Floor (op de grond --- on the floor/ground). Ground.)

Detailed word origin of grondig

Dictionary entry	Language	Definition
-ig	Dutch (nld)	-ed, having (when attached to a noun preceded by an adjective that describes the noun). -y; forms adjectives from nouns.
grond	Dutch (nld)	Floor (op de grond --- on the floor/ground). Ground.
grondig	Dutch (nld)	Thorough.

Words with the same origin as grondig