Cid as dech de shétaib?...

Cid as dech de shétaib?
Scían, ar is fri céill samailtir.

(what * that is * best * of * treasures?
knife * because * it is * to * intelligence * it is likened)

What is the best of treasures?
A knife, for it is a simile of intelligence.

This play on the keeness of blades and intellects comes from "Tochmarc Ailbe", where it is one of thirty riddles that Finn poses and Ailbe answers.