Perspicuously Perspicacious Perspectives

Blame The Scaffold and their 1968 UK hit 'Lily the Pink' for the title. I have no idea why that song popped into my head but pop it did. And having popped, I was obliged to silently sing along until I got to the last line of the chorus, when I found myself singing 'most perspicacious in every case'.

"Hang on a mo! That's not right," I interjected.

I had to go on-line in search of the lyrics where I was reminded that the correct word is 'efficacious'. Of course! 'Perspicacious' means...