origin: (1500's) Latin; efficāci-= effectual
adjective (noun: efficaciousness)
The opposite of faffing or screwing about; an antonym for an object that is rickety or faulty.
Something or someone that is "efficacious" is going to get the job done and as it should be done; efficient.
Example: Sometimes a good, hot bath is more efficacious than years of therapy.
The efficacious detective, Hercule Poirot, will solve any mystery.