Radak

For there is nothing straightforward in his mouth: – That is the reason why he says because of my watchful foes, for they are men of such a kind that there is no straightforwardness in the mouth (even) of one of them, but they make themselves appear to be friends. Their inward part is very wickedness: – meaning they are devising evil in their heart. An open sepulchre is their throat; – They make smooth their tongue: – for he who is not on his guard against them and listens to them when they make smooth their tongue and flatter and appear as friends will fall into their pit (sepulchre); and this is the intention of their heart, which is for evil. And he attributes (the uttering of) words to the throat, as (in the passage): “Let the high praises of God be in their throat” (Ps. 149:6), for some letters are pronounced in the throat. Further, all speech has its beginning from the throat, for the voice proceeds from the throat and makes speech possible to the mouth. Moreover, because the throat is open to the inside, which he compares to a sepulchre, he says their throat. And the word נכונה (straightforwardness) is a feminine adjective, but the substantive is wanting, and we must supply (in thought) אמרה (“utterance”) or דברה (“word”); and so “his meat (m.) is fat” [בראה fem., i.e. “a fat thing,” Hab. 1:16]: “like a flock (of ewes) that are shorn” [lit. “of the shorn ones,” הקצובות, Cant. 4:2]; “he answereth roughly” [lit. “rough things,” עזות; Prov. 18:23]; and similar passages.