After writing about words deriving from the root פלג, a reader asked if there was a connection to the word pilegesh פילגש - "concubine."
My first instinct was to consult Klein's Hebrew etymology dictionary, which did not make a connection to פלג, but had an interesting story nonetheless:
cp. Aram. פַלְקְתָא, Syr. פלקא (= concubine). cp. also Greek pallake, pallakis (= concubine). Avestic pairika (= beautiful women seducing pious men). All these words are certainly related, but it is difficult to establish the degree of their relationship to one another.
He also suggests a possible connection to the post-biblical Hebrew word palgas פלגס - which means "a sheep thirteen months old," and says that it derives from the Greek pallax, "youth, girl", which is a cognate of the Greek words pallake, pallakis that he mentioned in the pilegesh entry.
In his CEDEL entry for "Pallas" (another name for the Greek goddess Athene), he expands on this further. He says the name of the goddess comes from the Greek pallados, "maiden" which is cognate with pallake, pallakis - "concubine." After quoting some other Greek forms of the word, and the Avestic parika (quoted above), he mentions the Persian pari ("fairy", usually rendered as "peri" in English), and then suggests comparing with "Hebrew pileghesh, Aramaic pilaqta, 'concubine', and Arabic Bilqis, name of the queen of Sheba." And like his Hebrew entry, he isn't sure about how exactly these words are related. He notes that "the above cited Indo-European words are possibly Semitic loan words."
Whether Hebrew borrowed from the Greek, or if the Greek borrowed from a Semitic language, that would make pilegesh cognate with the metallic element palladium, which was named after the asteroid Pallas, which in turn was named for the Greek goddess.
Other suggest additional languages as the source of pilegesh. BDB mentions a possible Hittite origin, and in the footnotes of the Ben Yehuda dictionary, Tur-Sinai writes that it's possible that Egyptian is the source. Egyptian is also mentioned in the Theological Dictionary of the Old Testament (TDOT), which also quotes Chaim Rabin as concluding that the word is Philistine in origin (which could certainly have Greek influences.)
But surprisingly (at least to me) is that TDOT also proposes a connection to פלג:
We can give no satisfactory explanation for the origin of pileges (Greek pallax, pallakis; Latin pellex; Jewish-Aramaic palqeta; Syriac palqa; Arabic in the feminine proper name bilqis). Scholars have sought its home in both the Semitic and the Indo-European language families and have put forward many conjectures about mutual influence. Suggested etymologies include the Hebrew root plg, "divide, cleave," or a back-formation from Greek pallakis, pallake, pallax, originally "youth" or "girl," or from the same source plgs, "marriageable."
Gesenius, after quoting the same Greek and Latin words, writes:
The etymology is obscure, but the origin may be sought with some appearance of truth in the idea of softness and pleasure, with the Phoenicio-Shemitic roots פלג, פלק.
I don't really follow what he's writing here. First of all, I'm not familiar with the root פלק, and he doesn't include it in his dictionary, so maybe that was a typo. But I also don't see how to connect פלג with "softness and pleasure." And I don't see any mention of those terms in his entry for פלג, so I don't know where to go from there.
I took at look at Steinberg's Milon HaTanakh, who is usually happy to come up with a Hebrew origin for potentially foreign words, but he wasn't very helpful either. He does write that the Greek and Latin words we've mentioned were borrowed, via the Phoenicians, from Hebrew. But aside from rejecting Levita's idea that pilegesh comes from פלג-אשה (I guess "split-wife, half-wife"?), because it is "against the spirit of the language", he doesn't make a proposal of his own.
So to answer the initial question, I think that it doesn't seem too likely that pilegesh comes from peleg, but if it does, I'd need a better explanation (not just a guess) as to why.