The story
From being a weapon which produces heat (“thermo-”) and pressure (“baric”).
Etymology from Wiktionary, CC BY-SA 4.0.
thermobaric weapon
Today it means: A kind of bomb that works by dispersing an aerosol cloud of gas, liquid or powdered explosive, allowing chemical combustion to proceed using atmospheric oxygen. It produces an explosion, heat flash, shockwave, vacuum, and vacuum collapse pressure wave. Full definition →