Albert Barnes Commentary Psalms 17:10

"They are inclosed in their own fat: With their mouth they speak proudly." — Psalms 17:10 (ASV)

They are enclosed in their own fat. The meaning here is that they were prosperous, and consequently self-confident and proud, and were regardless of others. The phrase occurs several times as descriptive of the wicked in a state of prosperity, and therefore insensible to the rights, the needs, and the sufferings of others.

Compare Deuteronomy 32:15, “But Jeshurun waxed fat and kicked: thou art waxed fat, thou art grown thick, thou art covered with fatness; then he forsook God which made him,” etc. Job 15:27, “because he covereth his face with his fatness, and maketh collops of fat on his flanks.”Psalms 73:7, “their eyes stand out with fatness.”Psalms 119:70, “their heart is as fat as grease.”

With their mouth they speak proudly. This refers to speaking haughtily, in an arrogant tone, as a consequence of their prosperity.