ponson

English: Translation
A small, tapered steel tool used to drive the head of a finish nail flush with or below a wood surface, preventing hammer damage to the surrounding material. Syn. English: nail set, nail punch.
Notes

By extension: Any pointed instrument used to recess a fastener without marring the finished work.  

Etymology: Boie’nen pon- “to press, sink” + son “tip, point”, attested in 18th c. guild records for fine joinery.