godan verb (-su)
hammer out thinly; beat into a thin sheet
Used for metalworking, especially gold leaf or thin metal sheets.
職人が金箔を打ち延ばす。
The craftsman hammers out gold leaf.
Alternative kanji spelling; 伸ばす emphasizes stretching/extending.
延ばす is a general verb for extending or stretching something; 打ち延ばす specifically means to hammer something thin.
Compound of 打ち (uchi, 'striking') + 延ばす (nobasu, 'to stretch/extend'). The exact historical derivation is uncertain, but the meaning is transparent from its components.