groundly.

From Middle English groundli, growndly, grundlich (“firm, solid; deep, profound; fundamental, radical; of a person: learned, well-grounded”) [and other forms], from ground (“surface of the earth on which one stands; earth, soil; burial site; field of battle, battleground; dry land; flat land; piece of land; region; country; the earth; lowest part, base, bottom; foundation; platform; basis for a doctrine, opinion, etc.; fundamental principle; source; background (of a painting, etc.)”) (from Old English grund (“ground; bottom, foundation; abyss”), from Proto-Germanic *grunduz (“ground; foundation”), from Proto-Indo-European *gʰrem- (“to resound; to grumble, roar, thunder”)) + -li (suffix forming adjectives). The English word is analysable as ground + -ly (suffix forming adjectives), and is cognate with German gründlich (“careful, thorough; deep, profound”), Swedish grundlig (“thorough; in depth; profound; radical”).

From Middle English groundli, groundly, grundliche (“strongly, vehemently, violently; completely, fully, thoroughly; basically, fundamentally”) [and other forms], from ground (see etymology 1) + -li (suffix forming adverbs). The English word is analysable as ground + -ly (suffix forming adverbs).