Get to the precise point. Do or say something exactly right.
No one knows the exact origin of this phrase. What is known is that it is extremely old. It appears in The Book of Margery Kempe, circa 1438. This was an account of the life of religious visionary Margery Kempe and is considered to be the earliest surviving autobiography written in English:
“Yyf I here any mor thes materys rehersyd, I xal so smytyn ye nayl on ye hed that it schal schamyn alle hyr mayntenowrys.”
In modernised English, that reads as:
“If I hear any more these matters repeated, I shall so smite the nail on the head that it shall shame all her supporters.”
Kempe’s meaning in that citation isn’t entirely clear. Some have interpreted her ‘hit the nail on the head’ as ‘speak severely’. The current ‘get to the heart of the matter’ meaning is unambiguous in a later reference; William Cuningham’s The Cosmographical Glass, 1559:
You hit the naile on the head (as the saying is)