The story
From flash + damp, with 'damp' having the meaning of 'gas', as in old German, modern Dutch, and Danish.
Etymology from Wiktionary, CC BY-SA 4.0.
flashdamp
Today it means: A mixture of methane, carbon dioxide, and nitrogen that is found in mines, and ignited quickly within a safety lamp, but that is then extinguished quickly due to the exhaust products of the flame. Flashdamp can be heavier than air (called heavy flashdamp) or lighter than air (called light flashdamp), depending on the proportion of carbon dioxide. Full definition →