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 140:1The style of this Psalm resembles those of David in the former part of the book, presenting the usual complaint, prayer, and confident hope of relief. (Psa 140:1-13)
evil man--Which of David's enemies is meant is not important.
psa 140:2This character of the wicked, and the devices planned against the pious, correspond to Psa 10:7; Psa 31:13; Psa 58:4, &c.
psa 140:3sharpened . . . like a serpent--not like a serpent does, but they are thus like a serpent in cunning and venom.
psa 140:5snare [and] net--for threatening dangers (compare Psa 38:12; Psa 57:6).
psa 140:6(Compare Psa 5:1-12; Psa 16:2).
psa 140:7day of battle--literally, "of armor," that is, when using it.
psa 140:8(Compare Psa 37:12; Psa 66:7).
lest they exalt themselves--or, they will be exalted if permitted to prosper.
psa 140:9Contrasts his head covered by God (Psa 140:7) with theirs, or (as "head" may be used for "persons") with them, covered with the results of their wicked deeds (Psa 7:16).
psa 140:10(Compare Psa 11:6; Psa 120:4).
cast into the fire; into deep pits--figures for utter destruction.
psa 140:11an evil speaker--or, "slanderer" will not be tolerated (Psa 101:7). The last clause may be translated: "an evil (man) He (God) shall hunt," &c.
psa 140:12(Compare Psa 9:4).
psa 140:13After all changes, the righteous shall have cause for praise. Such
shall dwell--shall sit securely, under God's protection (Psa 21:6; Psa 41:12).