My heart is inditing - That is, I am engaged in inditing a good matter; though implying at the same time that it was a work of the heart - a work in which the heart was engaged. It was not a mere production of the intellect; not a mere work of skill; not a mere display of the beauty of song, but a work in which the affections particularly were engaged, and which would express the feelings of the heart: the result or effusion of sincere love. The word rendered is “inditing” - רחשׁ râchash - is rendered in the margin, boileth or bubbleth up. It means properly to boil up or over, as a fountain; and the idea here is that his heart boiled over with emotions of love; it was full and overflowing; it found expression in the words of this song. The Hebrew word does not occur elsewhere in the Bible.
A good matter - literally, a good word; that is, it was something which he was about to say which was good; something interesting, pure, important; not only a subject on which his heart was engaged, but also which was worthy of attention.
I speak of the things which I have made - literally, “I say my works to the king.” That is, My work - that which I meditate and am about to compose - pertains to the king.
Touching the king - He is to be the main subject of my song. Compare the notes at Isaiah 5:1. If the remarks made in the introduction to the psalm are correct, then the “king” here referred to was the future Messiah - the great personage to whom all the writers of the Old Testament looked forward, and whose glory they were so anxious to see and to describe. Compare the notes at 1 Peter 1:10-12.
My tongue is the pen of a ready writer - Let my tongue in speaking of him be as the pen of a rapid writer. That is, let my tongue rapidly and freely express my thoughts and feelings. The word rendered “pen” - עט ‛êṭ - means a stylus, usually made of iron, used for the purpose of inscribing letters on lead or wax. See the notes at Job 19:24. The idea is that the psalmist’s mind was full of his subject, and that he desired to express his thoughts in warm, free, gushing language - the language of overflowing emotion.