Concise Bible Dictionary:
This expression occurs in Romans 2:29; Romans 7:6 and 2 Corinthians 3:6, where the apostle contrasts it with “the spirit”; “the letter killeth, but the spirit giveth life.” Whether of the law or of the gospel, the mere intellectual reception of the words only leads to formality and death; it is only what is “of the Spirit” that can result in life. The Lord is the spirit of all that is written in letters in scripture.
Strong’s Dictionary of Hebrew Words:
Meaning:
(Aramaic) of Persian origin; an epistle (as carried by a state courier or postman)