Although the Saboraic sages connect “kiddushin” to “hekdesh,” Tosafos (to Kiddushin 2b) point out that the plain meaning of “kiddushin” relates to “Kiddush” in the sense of preparing or designating something (e.g., see Exodus 19:10 and Numbers 11:18). “Kedeishah” (a prostitute) is a cognate of this root because such a woman is set aside or designated for a specific purpose (see Rashi to Genesis 38:21 and Deuteronomy 23:18).
(Others believe “kedeishah” is related to the Akkadian word “qadistu,” which means a woman of special status. This definition fits both a prostitute and a wife.)
The commentators buttress Tosafos’s point with two arguments. First, when a groom performs kiddushin, he says to the bride, “You are mekudeshet to me…” Rabbi Avraham HaLevi of Barcelona (1235-1303) in Chiddushei HaRaah (Kiddushin 2b) and Rabbi Yosef Ibn Ezra (1560-1620) in Atzmot Yosef note that if “mekudeshet” meant consecration (as opposed to designation), the groom would effectively be consecrating her to himself, which would mean he would be forbidding her to himself, just like consecrating property renders it forbidden! Since the groom certainly does not mean to say this, it must be that “mekudesh” is a term of preparing or designating.
Second, Rabbi Eliezer Ashkenazi (1512-1585) argues that grammatically speaking, if the groom meant to “sanctify” her (as opposed to designate her), he would say “mukdeshet,” not “mekudeshet.”
Dr. Michael Satlow suggests that “kiddushin” is actually a loanword from the Greek legal term “ekdosis,” which refers to a bride’s father handing over his daughter to her husband. While this proposal is fascinating, it’s difficult to accept because handing over the bride is actually the definition of nissuin, not kiddushin (see Ketubot 4:5). Nevertheless, one could argue that the rabbis adopted this Greek word and slightly modified its pronunciation and meaning – as they often did when borrowing foreign words.
Let’s now turn our attention to “erusin” and its possible etymology. The Yemenite sage Rabbi Shalom Mansoura of Sanna (d. 1888) explains that “erusin” is an expression of tying (cf. the English euphemism for marriage, “tying the knot”).
Rabbi Yitzchak Ratzabi offers two explanations on this word’s etymology: First, he cites Rabbi Baruch HaLevi Epstein (1860-1940) who writes that the root of “erusin” (aleph-reish-sin/samech) should be understood as congruent to “erez” (aleph-reish-zayin) as the letters sin/samech and zayin are interchangeable. The latter root refers to something packed tightly (see Ezekiel 27:24) – hence the word “arizah” (package) in Modern Hebrew – so it makes sense that “erusin” would also refer to the powerful bonds of matrimony. Rabbi Epstein also notes that “erez” is related to “aizor” (tight belt).
Alternatively, Rabbi Ratzabi suggests that Rabbi Mansoura means that “erusin” is related to tying by way of a simpler metathesis without replacing any of the letters. If we simply transpose the final two letters of the root aleph-reish-sin/samech, we get aleph-sin/samech-reish, which means tie or bind. A betrothed woman is tied to her husband in the sense that the only way she can marry someone else is if he grants her a get (or dies).
Rabbi Yitzchak Vana (a Yemenite Kabbalist who died in 1670) argues that “arusah” (betrothed woman) is related to “eres” (poison) because once a woman is betrothed to another, she becomes like a poisonous snake or scorpion in the sense that anyone who illicitly approaches her is subject to the death penalty.
Rabbi Ratzabi cites another Yemenite scholar who explains that a betrothed woman is called an arusah in the same sense that a sharecrop is called an aris. The sharecrop enters a sort of partnership with the owner of the field who retains partial rights to its produce. Similarly, a betrothed woman enters into a partnership with her future husband, who at that point only has a partial claim over her (in that she is now forbidden from committing adultery), but not complete entitlement (i.e., if she dies, he does not inherit her property).
Finally, Rabbi Ratzabi offers two suggestions of his own on the etymology of “erusin,” both of which invoke the interchangeability of the letter aleph with ayin. The root ayin-reish-sin/samech refers to mixing – hence “arisah” (dough), which is mixed/kneaded. “Erusin” (spelled with an aleph) is also a mixture of sorts: the joining of man and wife in matrimony.
Alternatively, Rabbi Ratzabi connects “erusin” to “eres” (bed), spelled with an ayin, as an allusion to the conjugal reasons for marriage.