Noun
(UK, mining slang, obsolete) A large, round mass in a coal seam that forms part of the roof of a coal mine passage (usually being a fossilized root or stump); considered a hazard due to being prone to falling without warning.
Synonym: (vulgar) cauldron arse
Used other than figuratively or idiomatically: see cauldron, bottom.
Source: en.wiktionary.org