the.com/boiling point

means The exact temperature at which a liquid turns to vapor throughout its whole mass — and, figuratively, the moment someone's patience converts entirely into fury.

from A straightforward compound of 'boil' (from Latin 'bullire,' to bubble, via Old French 'boillir') and 'point' as in a precise threshold or mark. The literal physics sense — the temperature where vapor pressure matches the surrounding pressure — came first; the figurative use for human temper, where someone 'reaches their boiling point,' followed naturally once the phrase existed, since bubbling, agitated liquid was already an obvious stand-in for rising anger.