Let them melt away as waters which run continually - Let them vanish or disappear as waters that flow off, or floods that run by, and are no more seen. "Perhaps" the allusion here may be to the waters of a torrent that is swollen, which flow off and are lost in the sand, so that they wholly disappear. See the notes at Job 6:15-19. The prayer is, that his enemies might perish or be cut off, and that he might thus be saved from them.
When he bendeth his bow to shoot his arrows - literally, "he treads on his arrows." See the notes at Psalm 11:2. The meaning here is, When he prepares for an attack - or, prepares to make war, as one does who bends his bow, and places his arrow on the string. The allusion here is to the enemies of David, as seeking his life.
Let them be as cut in pieces - That is, Let his arrows be as if they were cut off or "blunted," so that they will produce no effect. Let them be such, that they will not penetrate and wound.

Let them melt away as waters - Let them be minished away like the waters which sometimes run in the desert, but are soon evaporated by the sun, or absorbed by the sand.
When he bendeth his bow - When my adversaries aim their envenomed shafts against me, let their arrows not only fall short of the mark, but he broken to pieces in the flight. Some apply this to God. When he bends his bow against them, they shall all be exterminated.

Let them (f) melt away as waters [which] run continually: [when] he bendeth [his bow to shoot] his arrows, let them be as cut in pieces.
(f) Considering God's divine power, he shows that God in a moment can destroy their force of which they brag.

Let them melt away as waters which run continually,.... Let them be disheartened, and their courage fail them, and let there be no spirit left in them, Joshua 7:5; or let them be unstable as water that is continually running, ever upon the flux and motion; let them never be settled, but always changing in their state and circumstances, Genesis 49:4; or let them "come to nought", as the Septuagint and Vulgate Latin versions; which is the case of water that runs over or runs away: or "let them be despised", as Jarchi, and the Arabic, Syriac, and Ethiopic versions; being useless and unprofitable, as water is when passed and gone: or let their ruin and destruction be as swift as the gliding water; let them be brought to desolation in a moment; Job 24:18; and let it be irrecoverable, as water running over the cup, and scattering itself, is spilled upon the ground, and cannot be gathered up, 2-Samuel 14:14. The Targum is,
"let them melt in their sins as water;''
when he bendeth his bow to shoot his arrows, let them be as cut in pieces; either when the wicked man bends his bow to shoot his arrows against the righteous; when he devises, his chief against him, shoots out bitter words, and attempts to do hurt unto him; let it be as if the string of his bow and his arrows were all cut to pieces; let all his designs, words, and actions, be without effect, and let not his hand perform his enterprise: or when God bends his bow against the wicked, so Jarchi; and prepares the instruments of death for them, and ordains his arrows against the persecutors, Psalm 7:12; let then his and his people's enemies be cut off, as the tops of the ears of corn; as the word used signifies, Job 24:24. The words may be rendered, "let him (God) direct his arrows; as the tops of the ears of corn are cut off" (f); so let them be.
(f) "concidantur, succidantur instar spicarum", Michaelis.

which run continually--literally, "they shall go to themselves," utterly depart, as rapid mountain torrents.
he bendeth . . . his arrows--prepares it. The term for preparing a bow applied to arrows (Psalm 64:3).
let them . . . pieces--literally, "as if they cut themselves off"--that is, become blunted and of no avail.

Melt away - As waters arising from melted snow, which at first run with great force, but are suddenly gone.