Truth being in the mouth represents speaking truthfully. Alternate translation: "For they never say what is true" (See: [[rc://en/ta/man/jit/figs-metonymy]])
The inward being represents people's thoughts and desires. Alternate translation: "their thoughts and desires are wicked" (See: [[rc://en/ta/man/jit/figs-metonymy]])
Their throat is spoken of as if it were an open tomb, ready for dead bodies to put into it. Possible meanings are 1) "they say that they will kill people" or 2) "What they say kills people" (See: [[rc://en/ta/man/jit/figs-metaphor]])