κάστωρ

The existence of Sanskrit कस्तूरी (kastūrī, “musk”), Sanskrit कस्तूरिका (kastūrikā, “musk-deer, musk”), and related terms points to reconstructing Proto-Indo-European *kestor- (“beaver, musk”) or a similar lemma. Note also Sanskrit कशीका (kaśīkā́, “weasel”), Sanskrit कश (káśa, “a type of rodent”), with etymologically suspect ś.

Beekes' assertion^[1] that the Sanskrit terms were derived from the Greek is absurd, given the attestation of the Indic terms not only in Classical Sanskrit, but also early Buddhist Pali and Jain Ardhamagadhi Prakrit, as well as India being the primary source of European musk. More plausible is that καστόριον (kastórion, “castoreum”) is a borrowing from Sanskrit, given its late attestation in Greek, although it is equally likely that the Greek terms were internally developed.

The traditional theory by Kretschmer derives the word from the name Κᾰ́στωρ (Kắstōr, “Castor”), who, in Greek mythology, was known as a savior of women, supposedly for the medicinal effect of castor fluid for women's diseases. Whether or not Castor was mentioned in relation to beavers in Greek mythology, the relationship between the two is presumably of Indo-European vintage, as reflected in Sanskrit नकुल (nakulá, “mongoose, weasel; Castor”), which similarly refers to both animal and the specific horse-twin. Beavers did not exist in Greece proper, as pointed out by some, but were nonetheless mentioned by Herodotus to exist in the North Pontic area, near the Proto-Indo-European homeland, so it is almost certain the Hellenic tribes were familiar with beavers before venturing into Greece.

A candidate for a native Pre-Greek word for "beaver" might be λᾰ́τᾰξ (lắtăx).

Note that the main Indo-European word for "beaver", *bʰébʰrus (which did not survive or exist in Hellenic), may be a reduplicative (and thus non-atomic) formation from *bʰerH- (“brown”).

While the Sanskrit derivation of καστόριον in Greek describing a type of musk exuded by beavers is plausible, the Greek Κάστωρ (Kastor), the name of a mythological character, is another matter.

In Mycenaean Linear B there is the man’s name Mycenaean Greek 𐀡𐀬𐀏𐀵 (po-ru-ka-to), assuming a typical nominative, ‘Polykastos’ (Πολύκαστος). An ending in ‑or is possible too. If Castor’s mythical twin Pollux, also known as Polydeuces, was literally very sweet, κάστωρ may have been adjectival and had a meaning other than sweet.