A Commentary, Critical, Practical, and Explanatory on the Old and New Testaments, by Robert Jamieson, A.R. Fausset and David Brown  at sacred-texts.com
psa 55:1In great terror on account of enemies, and grieved by the treachery of a friend, the Psalmist offers an earnest prayer for relief. He mingles confident assurances of divine favor to himself with invocations and predictions of God's avenging judgments on the wicked. The tone suits David's experience, both in the times of Saul and Absalom, though perhaps neither was exclusively before his mind. (Psa. 55:1-23)
hide not thyself, &c.--(compare Psa 13:1; Psa 27:9), withhold not help.
psa 55:2The terms of the last clause express full indulgence of grief.
psa 55:3oppression--literally, "persecution."
they . . . iniquity--literally, "they make evil doings slide upon me."
psa 55:4express great alarm.
psa 55:5come upon--or literally, "into."
psa 55:6be at rest--literally, "dwell," that is, permanently.
psa 55:7Even a wilderness is a safer place than exposure to such evils, terrible as storm and tempest.
psa 55:9Destroy--literally, "swallow" (Psa 21:9).
divide their tongues--or, "confound their speech," and hence their counsels (Gen 11:7).
the city--perhaps Jerusalem, the scene of anarchy.
psa 55:10which is described in detail (compare Psa 7:14-16).
psa 55:11Wickedness--literally, "Mischief," evils resulting from others (Psa 5:9; Psa 52:2, Psa 52:7).
streets--or literally, "wide places," markets, courts of justice, and any public place.
psa 55:12This description of treachery does not deny, but aggravates, the injury from enemies.
psa 55:13guide--literally, "friend" (Pro 16:28; Pro 17:9).
acquaintance--in Hebrew, a yet more intimate associate.
psa 55:14in company--literally, "with a crowd," in a festal procession.
psa 55:15Let death, &c.--or, "Desolations are on them."
let them go--literally, "they will go."
quick--or, living in the midst of life, death will come (compare Num 16:33).
among them--or, "within them," in their hearts (Psa 5:9; Psa 49:11).
psa 55:16God answers his constant and repeated prayers.
psa 55:18many with me--that is, by the context, fighting with me.
psa 55:19God hears the wicked in wrath.
of old--enthroned as a sovereign.
Because . . . no changes--Prosperity hardens them (Psa 73:5).
psa 55:20The treachery is aggravated by hypocrisy. The changes of number, Psa 55:15, Psa 55:23, and here, enliven the picture, and imply that the chief traitor and his accomplices are in view together.
psa 55:22thy burden--literally, "gift," what is assigned you.
he shall sustain--literally, "supply food," and so all need (Psa 37:25; Mat 6:11).
to be moved--from the secure position of His favor (compare Psa 10:6).
psa 55:23bloody . . . days--(compare Psa 5:6; Psa 51:14), deceit and murderous dispositions often united. The threat is directed specially (not as a general truth) against the wicked, then in the writer's view.