adj a. Extravagant; recklessly wasteful of one's property or means.
For what little of the mixed hardwood stands had survived..at the end of the seventeenth century had been laid waste by greedy and prodigal princelings.
Adjective: spending large amounts of money without thinking of the future, in a way that is not wise:
There have been rumours that he has been prodigal with company funds.
Although the meaning is a clear antonym for 'frugal', there may be an argument that it's being most commonly used in association with 'son' in reference to the parable of the prodigal son, creates a certain potential for confusion over the meaning in some quarters with people thinking it is to with returning after an absence rather than being spendthrift.