The translation of lovingkindness used in the King James Version is derived from the Coverdale Bible. This version was compiled by Myles Coverdale and based on the earlier translation by William Tyndale. The King James Version and later writers then compounded it into a single or hyphenated word. Although some feel that this word is now considered archaic, it is still part of the traditional rendition of Psalms in some English Bible translations and there are even modern worship songs still using the term.
Here's a selection of translations of the first few words of Psalm 51 across the centuries:
ἐλέησόν με ὁ θεός κατὰ τὸ μέγα ἔλεός σου (Greek)
Miserere mei, Deus, secundum misericordiam tuam (Vulgate)
God, haue thou merci on me; bi thi greet merci. (Wycliffe, 1388)
Haue mercy vpon me (o God) after thy goodnes (Coverdale Bible, 1535)
Haue mercie vpon mee, O God, according to thy louing kindnesse (KJV, 1611)
Have mercy upon me, O God, according to thy lovingkindness (KJV, 1769)
Have mercy on me, O God, according to thy steadfast love (RSV, 1952)
Be merciful to me, O God, because of your constant love (Good News, 1976)
Have mercy on me, O God, according to your steadfast love (NRSV, 1989)
Generous in love—God, give grace! Huge in mercy—wipe out my bad record (The Message, 1993)
And here's the New International Version translation, which I use most of all:
Psalm 51:1
Have mercy on me, O God, according to your unfailing love;
according to your great compassion blot out my transgressions.
I find it's always useful to see the same verse in different translations.







