Maul - A heavy sledge hammer. The word is connected with "malleus:" its diminutive "mallet" is still in use.

A man that beareth false witness against his neighbour,.... In whose house he has often been, and whom he has frequently visited; and, observing what was done there, not only discovers and tells abroad the secrets of his family, but even things which are false; yea, in a court of judicature, appears a witness against him, and swears falsely to his hurt and prejudice. Such a man
is a maul, and a sword, and a sharp arrow; that is, to his neighbour, against whom he bears false witness; and, by so doing, he mauls his fame, his credit, character, and reputation; and, as with a sword, takes away his life; and against whom there is no more guarding than against a sharp arrow, that comes from afar, suddenly and swiftly.

A false testimony is dangerous in every thing.

A false witness is as destructive to reputation, as such weapons to the body (Proverbs 24:28).
beareth . . . witness--literally, "answereth questions," as before a judge, against his neighbor.

This group of proverbs has the word רע in each of them, connecting them together. The first of the group represents a false tongue:

18 A hammer, and a sword, and a sharp arrow -
A man that beareth false witness against his neighbour.
An emblematic, or, as we might also say, an iconological proverb; for 18a is a quodlibet of instruments of murder, and 18b is the subscription under it: that which these weapons of murder accomplish, is done to his neighbour by a man who bears false witness against him - he ruins his estate, takes away his honour, but yet more: he murders him, at one time more grossly, at another time with more refinement; at one time slowly, at another time more quickly. מפיץ, from פּוּץ, is equivalent to מפּץ, and מפּץ from נפץ; the Syr. and Targ. have instead פדועא (פדיעא) from פּדע = פּצע; the word פּריעא, on which Hitzig builds a conjecture, is an error of transcription (vid., Lagarde and Levy). The expression, 18b, is from the decalogue, Exodus 20:16; Deuteronomy 5:17. It is for the most part translated the same here as there: he who speaks against his neighbour as a false witness. But rightly the lxx, Jerome, the Venet., and Luther: false testimony. As אל sA .y signifies both that which is mighty = power, and Him who is mighty = God, so עד signifies both him who bears testimony and the testimony that is borne, properly that which repeats itself and thereby strengthens itself; accordingly we say ענה עד, to give testimony in reply - viz. to the judge who asks - or generally to offer testimony (even unasked); as well as ענה לעד, Deuteronomy 31:21, i.e., as evidence (Jerome, pro testimonio). The prep. ב with this ענה has always the meaning of contra, also at 1-Samuel 12:3; Genesis 30:33 is, however, open to question.

A sword - Is as cruel and pernicious as any instrument of death.