Samech
118 The grace of God, then, being known, which alone frees from transgression, which is committed through knowledge of the law, he says, in prayer, “Fix with nails my flesh in Your fear”. For this some Latin interpreters have literally rendered the Greek καθήλωσον, which that language has expressed in one word. Some have preferred to render by the word confige, without adding clavis; and while they thus desire to construe one Latin by one Greek word, have failed to express the full meaning of the Greek καθήλωσον, because in confige nails are not mentioned, but καθήλωσον cannot be taken but of nails, nor can “fix with nails” be expressed without using two words in Latin....Hath he added, “For I have feared Your judgments”?
What means, “Fix me in Your fear: for I have feared”? If he had already feared, or if he was now fearing, why did he still pray God to crucify his flesh in His fear? Did he wish so much additional fear imparted to him as would suffice for crucifying his flesh, that is, his carnal lusts and affections; as though he should say, Perfect in me the fear of You; for I have feared Your judgments? But there is here even a higher sense, which must, as far as God allows, be derived from searching the recesses of this Scripture: that is, in the chaste fear of You, which abides from age to age, let my carnal desires be quenched; “For I have feared Your judgments,” when the law, which could not give me righteousness, threatened me punishment....For the inclination to sin lives, and it then appears in deed, when impunity may be hoped for.
But when punishment is considered sure to follow, it lives latently: nevertheless it lives. For it would rather it were lawful to sin, and it grieves that what the law forbids, is not lawful; because it is not spiritually delighted with the blessing of the law, but carnally fears the evil which it threatens. But that love, which casts out this fear, fears with a chaste fear to sin, although no punishment follow; because it does not even judge that impunity will follow, since from love of righteousness it considers the very sin itself a punishment. With such a fear the flesh is crucified; since carnal delights, which are forbidden rather than avoided by the letter of the law, are overcome by the delight in spiritual blessings, and also when the victory is perfected are destroyed.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)