Looking at the origin of the word 'because' I find it evolved from the phrase 'by cause', which was influenced by the French par cause de ; 'by cause' appeared in Middle English. What word was in use before, ex. in Old English?
Beowulf mostly uses some form of "forþon" (variations: forþan, forþam, etc.), though in a couple of places (such as lines 2638 and 2641) it uses "þe."
for is an old-fashioned-sounding synonym of "because": e.g.,
I dared not go outside, for it was raining.
But I am not sure if for was the direct predecessor of because.