8 letters · 16 points in Scrabble
noun a mixture of gases (mostly methane) that form in coal mines and become explosive when mixed with air
originFrom fire + damp with 'damp' not in the usual sense of 'moist' but old German, modern Dutch, Danish 'damp' as steam or vapour, hence a gas liable to catch fire.
Etymology from Wiktionary, CC BY-SA 3.0