This Mishnah talks about the number of times a person receives lashes. The Mishnah says, “How many times do we hit this person who is given Malkot? 40-1,” which is 39 times the person receives lashes. Shene'emar, like the Pasuk (verse) tells us, “He will strike him, he'll hit him, in a number of 40.” That phrase really means the number that will lead to 40, that will build up to 40. The number that will lead up to 40, which is 39. The number that's next to 40, which is 39 lashes. The fact that the Pasuk is phrased in that way tells us that it's not 40 exactly, but it's what is right before that.
Rabbi Yehuda argues with Tanna Kama, and he says, “No, a person is hit a full 40 times.” He takes the Pasuk at face value. It says 40, so he's given 40 lashes. Later on in the Perek (chapter), we'll learn that the lashes were given in groups of three: one on the chest and two on the shoulders. That would make sense according to the number 39. 39 is divisible by three. But according to the opinion of Rabbi Yehuda, who tells us that 40 lashes is a given, 40 can't be divided by three. How would he give that 40th lash? Where does he receive that extra 40th lash? Between his shoulders, on the top of his back.