Gunpowder placed in the pan of an early firearm to ignite the main charge after itself being ignited by friction, a spark, etc. Compare "priming powder".
Origin
Late 15th century. From touch + powder.
Definition of touch-powder in US English:
touch-powder
nounˈtʌtʃˌpaʊdə
rare, historical
Gunpowder placed in the pan of an early firearm to ignite the main charge after itself being ignited by friction, a spark, etc. Compare "priming powder".