dry powder

Short definition from Wiktionary

Noun

1. 1
   A powder found in a fire extinguisher that is expelled to smother the fire (in contrast with liquid types of extinguisher).
2. 2
   A powder found in an inhaler that delivers a bronchodilator for antiasthmatic or antiemphysemic purposes (indications).
3. 3
   Cash (or cash-like securities) kept in reserve in case of need.
4. 4
   Used other than figuratively or idiomatically: see dry, powder; especially, gunpowder kept dry to keep it from spoiling before use.