Literal: to hammer a nail again or to secure a nail more firmly by driving it further.
2.
SpainLatin Americacolloquialneutral
Figurative (emphasis): to reinforce or underscore a point by adding an additional remark or action that makes the original point more forceful.
3.
SpainLatin Americacolloquial
Figurative (negative consequence): to make a situation worse or to deliver a final damaging blow by adding an action or comment that exacerbates an existing problem.
Examples:
ES: En el cual, por remachar el clavo, has fallado.
EN: Which, to belabor the point, you failed.
ES: Y para remachar el clavo, se le cambia el lugar de nacimiento.
EN: And to top it off, they also change his birthplace.