The story
- Old English · inherited fromþǣrunder
- Middle English · inherited fromtherunder
- English todaythereunder
From Middle English therunder, þer-under, þar under, from Old English þǣrunder, equivalent to there + under. Cognate with Dutch daaronder (“thereunder”), German darunter (“thereunder”), Swedish därunder (“thereunder”).
Etymology from Wiktionary, CC BY-SA 4.0.