when a derabbanan is not a derabbanan

When I was learning Rosh HaShana with my son over Rosh HaShana, he noticed that Rashi (22a) seems to say that asmachta lo kanya (e.g. gambling is a form of theft) is a din derabbanan, contrary to the position of many Achronim. My son is in good company, because the Rashash noticed the same thing. Rashash tries to read Rashi as consistent with the other views -- when Rashi says, "Amur Rabbanan asmachta lo kanya..." Rashi doesn't mean that the Rabbis instituted the din of asmachta lo kanya; what Rashi means is that the Rabbis publicized and taught the din d'oraysa of asmachta lo kanya.

It seems strange that the statement, "The Rabbis said..." refers to a din d'oraysa -- where do you ever have such a thing? Actually, in a few places. The Mishna in Sukkah (41b) tells us that, "Amru Chachamim ain adam yotzei y'dei chovaso b'lulavo shel chaveiro b'Yom Tov rishon," that the Rabbis taught that a person does not fulfill his mitzvah on the first day of Yom Tov using someone else's lulav. The Mishna uses the term "amru Chachamim" for what is a din d'oraysa; the Mishna simply means that the Chachamim publicized the din. Rashash gives a few other examples to make the point.

Although it is not the exact same expression, a similar idea is found in another Mishna in R"H and Sukkah. The Mishna (R"H 30, Sukkah 41) says that R' Yochanan ben Zakai "hiskin" that new wheat could not be eaten on 16 Nisan. The gemara explains that hiskin here does not mean that R' Yochanan made a takanah derabbanan, but rather "darash v'hiskin," he publicized and taught what he held to be a din d'oraysa.