Deceive not with thy lips - Better, wilt thou deceive with thy lips?

Be not a witness - Do not be forward to offer thyself to bear testimony against a neighbor, in a matter which may prejudice him, where the essential claims of justice do not require such interference; and especially do not do this in a spirit of revenge, because he has injured thee before.

Be not a witness against thy neighbour without cause,.... Unless forced unto it, except there is some urgent reason for it; not upon any trivial account, or in any frivolous matter; never appear forward and eager to bear witness against him, and, whenever obliged to it, be not a false witness, but speak truth, whether thy neighbour be a friend or a foe;
and deceive not with thy lips; by bearing a false testimony, the judge, thy neighbour and thyself; for though men may be deceived, God cannot: or, shouldest thou do so, "thou wouldest break" and cut him to pieces "with thy lips" (x); which is the sense of the words according to R. Judah, as Ben Melech relates.
(x) "et ne atteras labiis tuis", Vatablus; "et ne comminuas eum labiis tuis", Syriac version.

There are three defaults in a witness pointed out.

Do not speak even truth needlessly against any, and never falsehood.

Warning against unnecessary witnessing to the disadvantage of another:
Never be a causeless witness against thy neighbour;
And shouldest thou use deceit with thy lips?
The phrase עד־חנּם does not mean a witness who appears against his neighbour without knowledge of the facts of the case, but one who has no substantial reason for his giving of testimony; חנּם means groundless, with reference to the occasion and motive, Proverbs 3:30; Proverbs 23:29; Proverbs 26:2. Other designations stood for false witnesses (lxx, Syr., Targ.). Rightly Jerome, the Venet., and Luther, without, however, rendering the gen. connection עד־חנם, as it might have been by the adj.
In 28b, Chajg derives והפתּית from פּתת, to break in pieces, to crumble; for he remarks it might stand, with the passing over of into , for והפתּות [and thou wilt whisper]. But the ancients had no acquaintance with the laws of sound, and therefore with naive arbitrariness regarded all as possible; and Bttcher, indeed, maintains that the Hiphil of פתת may be הפתּית as well as הפתּות; but the former of these forms with could only be metaplastically possible, and would be הפתּית (vid., Hitzig under Jeremiah 11:20). And what can this Hiph. of פתת mean? "To crumble" one's neighbours (Chajg) is an unheard of expression; and the meanings, to throw out crumbs, viz., crumbs of words (Bttcher), or to speak with a broken, subdued voice (Hitzig), are extracted from the rare Arab. fatâfit (faṭafiṭ), for which the lexicographers note the meaning of a secret, moaning sound. When we see והפתית standing along with בּשׂפתיך, then before all we are led to think of פתה [to open], Proverbs 20:19; Ps. 73:36. But we stumble at the interrog. ה, which nowhere else appears connected with ו. Ewald therefore purposes to read והפתּית [and will open wide] (lxx μηδὲ πλατύνου): "that thou usest treachery with thy lips;" but from הפתה, to make wide open, Genesis 9:27, "to use treachery" is, only for the flight of imagination, not too wide a distance. On וה, et num, one need not stumble; והלוא, 2-Samuel 15:35, shows that the connection of a question by means of ו is not inadmissible; Ewald himself takes notice that in the Arab. the connection of the interrogatives 'a and hal with w and f is quite common;
(Note: We use the forms âwa, âba, âthûmm, for we suppose the interrogative to the copula; we also say fahad, vid., Mufaṣsal, p. 941.)
and thus he reaches the explanation: wilt thou befool then by thy lips, i.e., pollute by deceit, by inconsiderate, wanton testimony against others? This is the right explanation, which Ewald hesitates about only from the fact that the interrog. ה comes in between the ו consec. and its perf., a thing which is elsewhere unheard of. But this difficulty is removed by the syntactic observation, that the perf. after interrogatives has often the modal colouring of a conj. or optative, e.g., after the interrog. pronoun, Genesis 21:7, quis dixerit, and after the interrogative particle, as here and at 2-Kings 20:9, iveritne, where it is to be supplied (vid., at Isaiah 38:8). Thus: et num persuaseris (deceperis) labiis tuis, and shouldest thou practise slander with thy lips, for thou bringest thy neighbour, without need, by thy uncalled for rashness, into disrepute? "It is a question, âl'nakar (cf. Proverbs 23:5), for which 'a (not hal), in the usual Arab. interrogative: how, thou wouldest? one then permits the inquirer to draw the negative answer: "No, I will not do it" (Fleischer).