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1.
The act of sitting on a hard surface when in dire need of a poo. Usually required when one finds themself far away from a toilet. Doing a gas bomb hopefully will allay the impending dump for a few minutes until you can find an appropriate recepticle. Grass is not a recommended surface as it is too soft.

Warning - A gas bomb performed incorrectly or at too late a moment may cause skidmarks.

Many gas bombs were commited on the low walls outside peoples houses in Charlton Kings, Cheltenham in the late 70's.
Person A: What are you sitting down?
Person B: I'm doing a gas bomb.
by Peter Reed February 17, 2008