Another one of the simanim is silka - סלקא. I had always assumed that silka meant beets, as in the Modern Hebrew word for beet - selek סלק.
So I was surprised to see that there are those that identify the silka with spinach. For example here:
Spinach is called Silka in Aramaic. Beets are called "Selek" in Hebrew, so either (or both) are fine.
Where did this understanding come from? One explanation about the Sefardim, as mentioned here, is that:
Their custom for the food "Silka" was cooked spinach, because the Arabic term for the word was "Salk."In fact, Rav Mordechai Eliyahu (the former Sefardi Chief Rabbi) here says not only is spinach preferable, but that the use of beet is more recent!
Stahl confirms that salk means both spinach and beets in Arabic. A Google comparison between "salk + spinach + arabic" (16,300 hits) and "salk + beet + arabic" (9,790 hits) shows an advantage to spinach. But it turns out that the Portugese word for beet - acelga - comes from the Arabic salk.
However, I have another theory for the identification of silka with spinach.
The gemara (Brachot 38b) mentions מיא דסלקא maya d'silka. Rashi explains this term as "water in which was cooked teradin." And in Eruvin 28b, raw teradin are equated with raw silka. What are these teradin תרדין? Every authoritative source I could find says that the tered mentioned here is beet (Rambam on Kilaim 1:3 [according to Kapach], Jastrow, Ben-Yehuda, Klein, Melamed).
However, in Modern Hebrew, tered means spinach! How this happened is not clear - perhaps due to the similarly looking leaves. Ben-Yehuda already complained about it. (He suggested a new word for spinach - kotzit קוצית - but like some other of his suggestions, it was not adopted.) So if someone was to read the talmudic passages above, they would likely believe that silka was spinach as well. (That is apparently the basis for this halachic question.)
What about the etymology of silka? Klein says that the etymology is unknown, but Jastrow has a reasonable explanation. He says it comes from סלק ( also שלק) which means "to boil down". Therefore silka originally meant a "well-boiled vegetable." (We've seen Jastrow's approach before - in regards to lefet - where the way a vegetable was prepared or eaten eventually gave the food its name.)
In regards to the prayer associated with silka, we say שיסתלקו אויבינו - "may our enemies depart, be removed". However, the verb סלק originally meant "to go up, ascend", and is related to the root נסק (from where we get masok מסוק - helicopter). To "ascend" seems perhaps too complimentary for our enemies. So if we call the beets tered instead of silka (or actually eat spinach, like the Sefardim), then our association can be with the root ירד (to descend) - perhaps "שתרד קבוצת אויבינו"...