To heat a liquid until it reaches a temperature where bubbles rapidly form and rise to the surface, or a painful infected bump on the skin.
From Old French 'boillir,' from Latin 'bullire' meaning 'to bubble,' connected to 'bulla' (bubble); the skin infection sense emerged in Middle English as a descriptive term for a bubble-like pustule.
The word 'boil' perfectly captures the action—it comes from the Latin 'bulla' meaning bubble, which is exactly what happens when water reaches 100°C and transforms from liquid to gas in visible bubbles!
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