אֱוִ֗יל בַּ֭יּוֹם יִוָּדַ֣ע כַּעְס֑וֹ וְכֹסֶ֖ה קָל֣וֹן עָרֽוּם׃
דעם נאַרנס ערגערניש װערט באַקאַנט אין אײגענעם טאָג, אָבער דער קלוגער פֿאַרדעקט די שאַנד.
Ramban
NEITHER SHALT THOU CONCEAL HIM — by not testifying against him in court. And in the Sifre it is written:163Sifre, R’eih 89. “Neither shalt thou spare — you shall not plead in his favor. Neither shalt thou conceal — if you know anything unfavorable to him, you are not permitted to stand mute.” All [these interpretations]172I.e., whether we interpret Neither shalt thou conceal by “not testifying” against him, or “withhold” anything unfavorable to him — the purport is the same, since covering up of evidence is a form of silence. convey the same thought, for the term “concealing” is from he that ‘concealeth’ his transgressions;173Proverbs 28:13. but a prudent man ‘concealeth’ shame,174Ibid., 12:16. meaning that he refrains from telling it. And he states, and they shall do no more any such wickedness as this is in the midst of thee,175Verse 12. because this matter [of a misleader] is very common, and it would be frequently done were it not for the law [as a deterrent, hence this pronouncement was necessary], but the matter of a false prophet mentioned [above] is not a common occurrence, [therefore, unlike in the verse before us, no admonitions were mentioned in Verse 6].
Kli Yakar
And according to this understanding, “your weapons” is like its plain meaning, a weapon, and in the letter vav [and] of and a stake [veyated] this entire exposition is hinted at. For every vav adds to the previous matter, telling you that protection from all evil speech of slander is to place one’s finger, which resembles a stake, into one’s ear — this is one’s weapon that saves from the scourge of the tongue. And Scripture says that similarly there is another stake in addition to the standard weapon, with which one digs in the earth to cover one’s excrement. So too, the vav extends to include a similar case: the finger that resembles a stake, with which one digs in the ear to make oneself as if not hearing, and thereby covers one’s shame and filth, as it is written and he who covers shame is prudent (Proverbs 12:16). The finger is an additional weapon beyond the standard weapons with which one fights in ordinary battles with the hands. But in a battle coming from the scourge of the tongue, nothing is better than hiding with one’s finger, and this is one’s weapon against the mouth that speaks slander and archers who shoot with the arrow of their sharpened tongue. There is no shield against them except silence. In this way, there is no need to change the word your weapons, for it is literally a weapon, and Scripture placed this protection next to the verse speaking about covering excrement because they are both the same matter.
Rashi
כשית — There exists a Kal form (as is this word) in the sense of covering, as (Proverbs 12:16) “And a prudent man is covered with (וְכֹסָה) shame”. If it had written כִּשִּׂיתָ with a dagesh in the ש. it would imply, “Thou hast covered others (or other things)” (i.e., it would be transitive), as in the text already quoted, “For he hath covered (כִסָּה) his face”.