The Talmud discusses the prohibition of affectionate contact with a nidah. According to the Rambam (Hil. Issurei Biah 21:1, Sefer HaMitzvot, prohibitions 353), such behavior is biblically prohibited, on the strength of the verse “you shall not approach …to uncover her nakedness” (Vayikra 18:19), and thus incurs a punishment of lashes. The Ramban, in disagreeing, offers two alternate possibilities: a) the prohibition is rabbinic; b) the prohibition is indeed biblical, but without the usual punishment of lashes, “like chatzi shiur” (an amount of prohibited food less than that required for punishment, but nonetheless biblically prohibited).

Some later commentaries have understood the Ramban’s intent to be that not only does a notion of “chatzi shiur” exist on a quantitative level, prohibiting actions involving objects that fall short of minimum measurements, but also on a qualitative level, prohibiting actions that approach transgressions while not actually crossing that line. (See Resp. L’Horot Natan, I, 57, noting the Rambam’s words in Hil. Ma’akhalot Asurot, 3:6; note also Resp. Eretz Tzvi, I, 1, fn. on page 56).

It is possible, though, to understand the Ramban’s intent differently. It may be that he was not suggesting that contact with an ervah is a “chatzi shiur” of the prohibition of intimacy, and thus prohibited, but rather that chatzi shiur provides a model for the category: biblically prohibited, related to a punishable offense, but yet lacking a punishment itself.

Assuming the prohibition is biblical, it would seem to fall into a category of prohibitions that are rooted in the Torah, but are in intent similar to rabbinic prohibitions, as they are protective in nature. R. Yosef Engel (Lekach Tov, 8), in an essay on the topic, lists 17 such examples. This particular prohibition is especially relevant as it is identified by the Avot D’Rav Natan (ch. 1) as an example of such a notion (in interpretation of the mishnah , Avot 1:1 : “create a safeguard for the Torah”).