Why didn't eliminating 'þon þy' eliminate the meaning of 'for' as 'because'?
Etymonline doesn't expound how:
for þon þy shortened to only 'for'.
'for' persisted in meaning because. 'For' is a function word that means many meanings, and 'for' alone doesn't intutively mean 'because'.
To wit, in omitting þon þy in for þon þy, you're eliminating the content words that impart the meaning of "because" to for þon þy. But why didn't eliminating "þon þy" eliminate the meaning of "because" in 'for' too?
For alone as a conjunction, "because, since, for the reason that; in order that" is from late Old English, probably a shortening of common Old English phrases such as for þon þy "therefore," literally "for the (reason) that."
Solution 1:
A not insignificant role might have played forth. The first to quotes in wiktionary on OE forth do looketh like a fixed collocation that fell out of use and appeared in context of entailment. My first thought had been that an univerbation forth-thon (thy) might simply loose the ending of thon under the setting-in erosion of middle english morphology. The second thought is, that it might appear like for it, which is a frequent collocation now. So I went forth, didn't exactly find confirmation, but see for yourself:
forþ
- expresses the continuation of an action
late 10th century, Ælfric, "On the Festival of St. Peter the Apostle"
Petrus cnocode forþ oþ þæt hīe hine inn lēton.
Peter kept knocking until they let him in.
late 10th century, Ælfric, "On the Festival of Saint Peter the Apostle"
Hīe ēodon forþ oþ þæt hīe cōmon tō ānum wīċe.
They kept walking until they came to a street.
9th century, Bald's Leechbook vol. I
Drince hē forþ þone drenċ fēowertīene niht.
He should continue to drink the potion for fourteen days.
You'll note that "until" can be well replaced with "for" in the first example. It's not obvious why that should occur; after all, forth still exists. It has lost this meaning particular meaning, but one would expect the morpheme had been recognizable--in contracted speech it might have been confused, perhaps.
It is by the way notable that oth thaet had carried most of the conjunctive force, which neatly parallels German auf dass "for that, so that" (wherein auf "up, upon" might have been informed by ob, if, insofar there can be no notion of unbroken descent in view of the wide divergence between conjunctions and prepositions in the various Germanic lects). I cannot find an etymology, nor a continuation, for oth "on, until" right now, only an instrumental suffix -oth, and of course until, which has a cognate in the Hittite language (for example) and in English and.
It goes without saying that close and distant relatives of for had often been used in conjunctive phrases, though I'm not aware of it being used alone like that.
Of course, if it was all an accident, the explanation would have to be more involved. Vice versa, if there's no