Albert Barnes Commentary Psalms 109:18

"He clothed himself also with cursing as with his garment, And it came into his inward parts like water, And like oil into his bones." — Psalms 109:18 (ASV)

As he clothed himself with cursing, as with a garment - Moral qualities are often compared with clothing - as that in which we “appear” to others. See 1 Peter 5:5; Job 29:14.

So let it come into his bowels like water - Margin, “within him.” Hebrew, “In his midst.” Let it penetrate him through and through. Let no part of him be unaffected by it.

And like oil into his bones - As if oil flowed through all his bones, so let the effects of cursing pervade his whole frame. The prayer is, that his entire nature might feel the effects of cursing; that he might know to the full what he was endeavoring to bring on others.