My eye grows weak because of all my FOES

Watch the parallelism in Ps 6:

6 I am weary with my moaning;
every night I flood my bed with tears;
I drench my couch with my weeping.

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David cried to sleep. His anguish was taxing on his eyes. The next verse reinforced the last one.

7 My eye wastes away because of grief; it grows weak because of all my foes.

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His grief stressed his eyes. His enemies caused his weeping. It was taking a physical toll on his eyes.

His foes caused him to weep on his bed. This nightly routine was weakening his eyes physically. It's a vivid expression of how deep emotional pain could manifest physically, and it's part of the psalmist's appeal to God for deliverance and healing.

8 Depart from me, all you workers of evil, for the Lord has heard the sound of my weeping. 9 The Lord has heard my plea; the Lord accepts my prayer. 10 All my enemies shall be ashamed and greatly troubled; they shall turn back and be put to shame in a moment.

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