before 1000; Middle English as, als, alse, also, Old English alswā, ealswā all so (see also), quite so, quite as, as; cognate with Middle Dutch alse (Dutch als), Old High German alsō (Middle High German álsō, álse, als, German also so, als as, as if, because)

8. See because.

As a conjunction, one sense of as is “because”: As she was bored, Sue left the room. As also has an equally common use in the sense “while, when”: As the parade passed by, the crowd cheered and applauded. These two senses sometimes result in ambiguity: As the gates were closed, he walked away. (When? Because?)
As … as is standard in both positive and negative constructions: The fleet was as widely scattered then as it had been at the start of the conflict. Foreign service is not as attractive as it once was. So … as is sometimes used in negative constructions (… not so attractive as it once was ) and in questions ( “What is so rare as a day in June?” ).
The phrase as far as generally introduces a clause: As far as money is concerned, the council has exhausted all its resources. In some informal speech and writing, as far as is treated as a preposition and followed only by an object: As far as money, the council has exhausted all its resources.
As to as a compound preposition has long been standard though occasionally criticized as a vague substitute for about, of, on, or concerning: We were undecided as to our destination. As to sometimes occurs at the beginning of a sentence, where it introduces an element that would otherwise have less emphasis: As to his salary, that too will be reviewed. As to what and as to whether are sometimes considered redundant but have long been standard: an argument as to what department was responsible. See also all, farther, like¹, so¹.

Borrowed into English from Latin around 1595–1605