Adam Clarke on Psalms 86:15

But thou, O Lord - What a wonderful character of God is given in this verse! אדני Adonai, the Director, Judge, and Support; - but instead of אדני Adonai, thirty-four of Kennicott's MSS. have יהוה Jehovah, the self-existent and eternal Being; - אל El, the strong God; רחום rachum, tenderly compassionate; חנון channun, the Dispenser of grace or favor; ארך אפים erech appayim, suffering long, not easily provoked; רב חמד rab chesed, abundant in blessings; and אמת emeth, faithful and true. Such is the God who has made himself more particularly known to us in Christ. The scanty language of our ancestors was not adequate to a full rendering of the original words: "And thu driht God gemildsiend, and mildheort, gethyldig and mucel mildheortnysse and sothfaest - And thou, Lord God, art mild, and mildhearted, patient, and of much mildheartedness, and soothfast," - steady in truth.

In the old Psalter the language is but little improved: And thou Lorde God mercier, and mercyful, sufferand, and of mykel mercy, and sothefast.

The word mercier is interpreted, do and dede of mercy.