hollow

From Middle English holow, holwe, from Old English holh ("a hollow"), tracing back through Proto-West Germanic *holh and Proto-Germanic *hulwiją to *hulaz, and ultimately to the Proto-Indo-European root *ḱel-, meaning "to cover, hide." Old High German huliwa and hulwa, and Middle High German hülwe, are cognates along the way.

The word began life as a noun in Old English meaning simply "a hollow," a hidden or covered-over space, before it settled into use as an adjective describing anything with empty space inside.

It's directly related to hole, which shares its Old English root holh. The word holler is worth comparing too, though the facts here don't spell out exactly how the two connect.