“Avak Lashon HaRa” is “the dust of Lashon HaRa” and is something which isn’t outright Lashon HaRa, but it implies Lashon HaRa.
 The gemara in maseches “Chullin” notes the different ways that the shechitah (slaughtering) knife was checked. According to the gemara, “Rav Sheishess checked [the shechitah knife] with the tip of his tongue.”
Apparently, based on this section of “Shmiras HaLashon” and the fact that Rav Sheishess specifically used his tongue to examine the shechitah knife, it is evident that the tongue is the most sensitive of all of the organs and would be most prone to discover any knicks on the knife that would make it unusable for shechitah.
 The text of the gemara is as follows: “Shmuel said to Rav Yehuda: ‘Sharp one, open your mouth and read, open your mouth and learn, so that your [Torah learning] will remain within you and [that] you will prolong your life, as it says, “For they (i.e. the wisdom of The Torah – Metzudas David) are life for those who find them and it [provides] healing for all of his flesh.” (Mishlei: 4; 22) Do not read it [as] “those who find them” – “למוֹצאי-הם”, rather, [read it as], “those who [utter] it from their mouths” – “למוציאי-הם בּפּה”.”