the part of a hammerhead opposite the flat striking surface (may have various shapes)
The (often spherical) end of the head of a hammer opposite the main hammering end.
Penis.
To shape metal by striking it, especially with a peen.
The word "peen" likely comes from a North Germanic source. Its original meaning was related to pounding iron with a hammer, as seen in dialectal Norwegian and Old Swedish terms for the same action.