en_tn_condensed/psa/144/008.md

Their mouths speak lies

Here the word "mouths" represent the people who speak. Alternate translation: "They speak lies" (See: rc://en/ta/man/translate/figs-synecdoche)

their right hand is falsehood

Possible meanings are 1) David speaks of the custom of raising the right hand to swear that what one is about to say in court is true, "they lie even when they swear to tell the truth," or 2) the "right hand" is a metaphor for power, "everything they have gotten, they have gotten by telling lies." (See: rc://en/ta/man/translate/figs-metaphor)