BT Zev 19a(34) :מתה ודאי חיצא חיה מאי כיון דקא וקא אתיא לא חייצא או דלמא כיון דקפיד עלה חיצא‏ : (if the louse) is dead it definitely interposes (between the garment and the flesh). What if it is alive? Since it moves around [lit. goes and comes] it does not interpose. Or perhaps since he is particular about it, it interposes
See the entire entry for ḥṣṣ#3 vb. to interpose, come in between