... folk started in Germanic as a word referring to a division of an army, and only later morphed into meaning a tribe or a nation. The Proto-Germanic word was fukla; the early Semitic root for divide -- i.e., as in making a division -- was p-l-kh:
In the early Semitic language Assyrian, that root was used to mean district (i.e., a division of land), with the kh softening into a g (puluggu). In Hebrew today, a detachment is a plaga. Maybe in Northern Europe, that root came out as fulka in the same meaning.
The Online Etymology Dictionary has a different theory about the origin of "folk":
Old English folc "common people, laity; men; people, nation, tribe; multitude; troop, army," from Proto-Germanic *fulka- (source also of Old Saxon folc, Old Frisian folk, Middle Dutch volc, Dutch volk, Old High German folc, German Volk "people"). Perhaps originally "host of warriors:" Compare Old Norse folk "people," also "army, detachment;" and Lithuanian pulkas "crowd," Old Church Slavonic pluku "division of an army" (hence Russian polk "regiment"), both believed to have been borrowed from Proto-Germanic. Old English folcstede could mean both "dwelling-place" and "battlefield." According to Watkins, from PIE *ple-go-, suffixed form of root *pele- (1) "to fill," which would make it cognate with Greek plethos "people, multitude," and Latin plebes, "the populace, the common people." Boutkan thinks both the Germanic and Balto-Slavic could be a common borrowing from a substrate language.
The entry makes no mention of a Semitic connection. However, it does keep most of the cognate words to "folk" in the Germanic language family, and after quoting Watkins' theory about a connection to plethos and plebes (a theory which Klein rejects in his CEDEL, presumably because "people" and "multitude" were not the same as a division of warriors), there is mention of a "substrate language." That term refers to a language that influences another language by contact - which is exactly what McWhorter and Vennemann are saying that a Semitic language was in this case.
Whether or not they are the source of "folk", the two related Semitic roots that McWhorter mentioned - פלח (p-l-kh) and פלג (p-l-g) - gave us many Hebrew words.
The root פלח originally meant "to cleave, split", as McWhorter mentioned. That meaning is maintained in Hebrew in the word פֶּלַח pelach, meaning "section, slice of fruit." But from there it developed into the specific sense of "to plow, till the ground." Arabic has a cognate to this meaning, falahah, which led to fallah "plowman", the source of the word for peasant, "fellah", which has entered into English. (No connection to "fellow", though.)
From working and serving the land, פלח expanded to a more religious meaning of divine worship, similar to how the root עבד can indicate both working the land and worshipping God (or "cultivate" and "cult" in English). This sense is most commonly seen in the word (originally from Aramaic) פולחן pulchan - "service." Pulchan originally was any kind of service, then became religious service / divine worship, but in Modern Hebrew it has returned to a more secular meaning, of any kind of ritual indicating extreme admiration and devotion (like a cult).
The cognate פלג provides even more words. As with פלח, the root means "cleave, split, divide." Here are a sample of some of the words deriving from that root:
- פִּלֵּג pileg - "to divide, separate"
- הִפְלִיג hiflig - "to depart (by ship), to set sail"
- התפלג hitpaleg - "to split"
- פֶּלֶג peleg - "section, faction", also "brook, tributary"
- פְּלֻגָּה pluga - "army division" specifically a "company"
- פְּלֻגְתָּא plugta - "disagreement, argument". This is from Aramaic, and has a literary connotation, and is commonly found in the phrase בַּר-פְּלֻגְתָּא bar plugta - "scholarly opponent."
- מִפְלָגָה miflaga - "political party"