Their heart is as fat as grease - They are prospered. They have health, property, influence, comforts of all kinds. heaven appears to smile upon them, and it seems as if it were one effect of a wicked course of life to make people prosperous. See Psalm 17:10, note; Psalm 73:7, note.
But I delight in thy law - Though its observance should not be attended by any such results as seem to follow wickedness, though I am poor, emaciated, pale - disappointed, slandered, persecuted - though my lot in life is among the lowly and the despised - yet I will adhere to my purpose to keep thy law. It is, and it shall be, my delight, whatever may be the effects of so observing it. See Psalm 119:35.

Their heart is as fat as grease - They are egregiously stupid, they have fed themselves without fear; they are become flesh-brutalized, and given over to vile affections, and have no kind of spiritual relish: but I delight in thy law - I have, through thy goodness, a spiritual feeling and a spiritual appetite.

(c) Their heart is as fat as grease; [but] I delight in thy law.
(c) Their heart is indurate and hardened, puffed up with prosperity and vain estimation of themselves.

Their heart is as fat as grease,.... Or tallow, a lump of it, fat or grease congealed. That is, the heart of the above proud persons, who abounded in riches, were glutted with the things of this world; had more than heart could wish, and so became proud and haughty: or their hearts were gross, sottish, senseless, and stupid, as persons fat at heart are; or as creatures over fat, which have little or no feeling: so these had no knowledge of the law of God, no sense of their duty, no remorse of conscience for sin; their hearts were hardened, and they past feeling, and given up to a reprobate mind; see Isaiah 6:9; The Targum is,
"the imagination of their heart is become gross as fat:''
the Septuagint is, "curdled like milk"; that is, hardened, as Suidas (s) interprets it;
but I delight in thy law; after the inward man; as the apostle did, Romans 7:22; as fulfilled in Christ; as in his hands, as King and Lawgiver; as written upon his own heart; and so yielding a ready and cheerful obedience to it; he delighted in reading the law, in meditating on it, and in observing it.
(s) In voce

fat as grease--spiritually insensible (Psalm 17:10; Psalm 73:7; Isaiah 6:10).

Fat - They are stupid and insensible.