Alternative forms 
From alteration of earlier bunny (“lump, swelling”), from Middle English bony, boni (“bunion, swelling”); or perhaps from Italian bugnone (“a knob, boil, blain”), an augmented form of Italian bugno (“beehive”); both Middle English and Italian words from Old French bugne, buigne, bune (“bump, knob, swelling”), from Old Norse bunga (“an elevation, bulge”) or from Old Frankish *bungjo (“a swelling, lump, bump”), both Old Norse and Old Frankish from Proto-Germanic *bungô, *bunkô (“lump, clump, heap, crowd”), from Proto-Indo-European *bʰenǵʰ- (“thick, dense, fat”). Cognate with Dutch bonk (“lump, clump”), German Bunge (“swelling, tuber”).
bunion (plural bunions)
- (pathology) A bump or bulge on the first joint of the big toe caused by the swelling of a sac of fluid under the skin.