Probably onomatopoeic, imitating the sound of bubbling water — independently formed across several Germanic languages.
A thin sphere of liquid enclosing air or gas.
Probably imitative in origin — the word mimics the sound of bubbling water. Related to Middle Dutch 'bobbel' and Swedish 'bubbla,' all likely independent onomatopoeic formations. Key roots: bobel (Middle English: "bubble (imitative)").
The South Sea Bubble of 1720 gave 'bubble' its financial meaning — an inflated economic speculation that bursts dramatically.