When attacking, the unit expends a gold bar in their inventory to increase the damage they deal, as well as reduce the damage they take. Learned by Merchants at level 15 or higher. Like passive damage boosting skills (e.g weaponfaire skills), Spendthrift is calculated in the combat forecast.
Spendthrift can be considered a safer version of Life and Death, as the user does not have to worry about increased damage from the foe. Interestingly enough, both Spendthrift and Life and Death stack with each other, allowing the user to deal 20 additional damage at no penalty to their defense.