I wrote this post over ten years ago. It has been slightly edited to make sense for this blog.
I have been reviewing John Meier's work entitled A Marginal Jew and I am trying to share thoughts with you while working my way through the book. On pp. 221-222, Meier writes:
"We have no clear evidence that the famous passage of Isa 7:14 cited by Matthew ('behold, a virgin shall conceive') was ever taken to refer to a virginal conception before NT authors used it. The Hebrew text refers simply to a woman called an 'ALMA^, a young woman of marriageable age. Even the [LXX] of Isa 7:14 need not refer to virginal conception. While PARQENOS, the word the [LXX] uses to translate 'ALMA^, does often mean 'virgin,' it can also carry the more general meaning of a young girl of marriageable age and is so used at times in the [LXX]."
Admittedly, one has to look at the extensive footnotes in Meier's monograph to comprehend his line of thought fully here. Is there solid linguistic evidence for rendering Isa 7:14 with the term "virgin"? Frankly, I have no objection to the NWT rendering of Isa 7:14 ("maiden"), but it seems that 'ALMA^ could very well refer to a virgin.
I think you make a good point about the Hebrew word OTH ("sign"). OTH is often used to delineate wondrous works (i.e., miracles) of YHWH in the OT. The appearance of OTH in Isa 7:14 is thus a good indicator that something was unusual or supernatural about the "maiden" conceiving ('bearing a son').
I guess the only problem with translating it "virgin" is the historical circumstances surrounding the prophecy of Isaiah. It is quite possible that a married woman, namely, Isaiah's wife, fulfilled the prophecy in ancient Judah. We cannot be dogmatic about the identity of the "maiden" in ancient times, but the point is that she probably did not conceive and give birth to a son while a virgin. Nevertheless, I think the semantic range of 'ALMA^ certainly allows for the translation "virgin." The prophet could have actually used the term 'ALMA^ in an ambiguous way since his words were ultimately propleptic in nature. That is, he could be talking about a young girl of marriageable age in the minor fulfillment and a literal virgin in the antitypical fulfillment of this prophecy. But I tend to lean toward the "virgin" shade of meaning for the word 'ALMA^.
To be fair to Meier, PARQENOS does not necessarily refer to a virgin. But the synchronic evidence suggests that the LXX translators had a literal virgin in mind when they used PARQENOS. LSJ has this information on PARQENOS:
parqe/nos, Lacon. parse/nos Aristoph. Lys. 1263 (lyr.). h(, maiden, girl, Hom. Il. 22.127, etc. ; ai( a)/qliai p. e)mai/ my unhappy girls, Soph. OT 1462, cf. Aristoph. Kn. 1302 ; also gunh\ parqe/nos Hes. Th. 514; p. ko/ra, of the Sphinx, dub. in Eur. Phoen. 1730 (lyr.); quga/thr p. Xen. Cyrop. 4.6.9 ; of Persephone, Eur. Hel. 1342 (lyr.), cf. S.Fr.804; virgin, opp. gunh/, IDEM=Soph. Trach. 148, Theoc.27.65.
2. of unmarried women who are not virgins, Hom. Il. 2.514, Pind. P. 3.34, Soph. Trach. 1219, Aristoph. Cl. 530.
3. *parqe/nos, h(, the Virgin Goddess, as a title of Athena at Athens, Paus. 5.11.10, au=Paus. 10.34.8 (hence of an Att. coin bearing her head, E.Fr.675); of Artemis, Eur. Hipp. 17 ; of the Tauric Iphigenia, Hdt. 4.103 ; of an unnamed goddess, SIG46.3 (Halic., v B.C.), IG12.108.48,au=IG 12.108.54=lr (Neapolis in Thrace); ai( i(erai\ p., of the Vestal Virgins, D.H.1.69, Plu.2.89e, etc. ; ai( *(estia/des p. IDEM=Plu.Cic.19; simply, ai( p. D.H.2.66.
4. the constellation ti=D.H. Virgo, Eudox.ap.Hipparch. 1.2.5, Arat.97, etc.
5. = ko/rh III, pupil, X.ap.Longin.4.4, Aret. SD1.7.
II. as Adj., maiden, chaste, parqe/non yuxh\n e)/xwn Eur. Hipp. 1006, cf. Porph. Marc.33 ; mi/trh p. Epigr.Gr.319 : metaph., p. phgh/ Aesch. Pers. 613.
III. as masc., parqe/nos, o(, unmarried man, Apoc.14.4.
IV. p. gh= Samian earth (cf. parqe/nios III), PMag.Berol.2.57.