1
general::
Work out with considerable effort, as in It took weeks of negotiations to hammer out an acceptable compromise. This usage likens intellectual effort to shaping metal with the blows of a hammer. [Mid1700s]
American Heritage Idioms
2
general::
Phrase(s): hammer something out
1. Lit. to hammer a dent away; to make a dent even with the surrounding surface. • I’m going to have to have someone hammer this dent in my fender out. • It will take a while to hammer out the dent. 2. Lit. to expand something by hammering it thinner. • He hammered the gold out into a very thin sheet. • He hammered out the gold into thin sheets. 3. Fig. to arrive at an agreement through argument and negotiation. • The two parties could not hammer a contract out. • At last, we were able to hammer out an agreement. 4. Fig. to play something on the piano. • She hammered the song out loudly and without feeling. • Listen to John hammer out that song on the piano.
McGrawhill's American Idioms And Phrasal Verbs