To finalize something definitively or to determine something with certainty and precision.
This phrase derives from carpentry, where nailing something down makes it secure and immovable. The metaphorical use emerged in American English in the late 19th century, extending the idea of physical securing to abstract concepts like agreements or facts.
The carpentry origin gives this phrase a satisfying sense of permanence and craftsmanship. When you 'nail something down,' you're not just deciding - you're building something solid that won't come loose, much like a well-constructed house.
Complete word intelligence in one call. Free tier — 50 lookups/day.