Albert Barnes Commentary Psalms 140:2

"Who devise mischiefs in their heart; Continually do they gather themselves together for war." — Psalms 140:2 (ASV)

Which imagine mischiefs in their heart - Here the language is changed to the plural number in the Hebrew, implying that while there was one man who was eminent in his wickedness and his wrongdoing, there were many others associated with him, acting under his direction. The word “mischiefs” in the Hebrew means “evils; wickednesses.” It was not a single purpose; the plan embraced many forms of evil—doing him wrong in every way possible.

Continually are they gathered together for war - They are organized for this purpose; they are constantly prepared for it. The word rendered “gathered together” properly means to sojourn, to dwell for a time; and it has been proposed by some to render this, “All the day they dwell with wars;” that is, they are constantly involved in them. But the word may mean also “to gather together,” as in (Psalms 56:6).