Striking repeatedly with a hammer, or doing anything forcefully and repeatedly; to hammer home an idea means to emphasize it strongly.
From Old English 'hamer' (hammer), likely from Proto-Germanic roots. The '-ing' gerund form shows the continuous action of striking or pushing.
Humans have been hammering for over 3 million years—the stone tools our ancestors used worked by hammer strikes—so this might be one of the oldest 'verbs' in human behavior, even before we had words for it.
Complete word intelligence in one call. Free tier — 50 lookups/day.