dry powder

The financial sense comes by metaphor from the gunpowder sense.

dry powder (countable and uncountable, plural dry powders)

1. A powder found in a fire extinguisher that is expelled to smother the fire (in contrast with liquid types of extinguisher).
2. A powder found in an inhaler that delivers a bronchodilator for antiasthmatic or antiemphysemic purposes (indications).
3. (finance) Cash (or cash-like securities) kept in reserve in case of need.

   Private equity firms were sitting on more than $1 trillion in dry powder at the end of 2011.

4. Used other than figuratively or idiomatically: see dry,‎ powder; especially, gunpowder kept dry to keep it from spoiling before use.

• dry powder inhaler

• (stay prepared; maintain reserves): keep one's powder dry