always:  In Old English, the expression was alne weg, literally ‘all the way’. It seems likely that this was used originally in the physical sense of ‘covering the complete distance’, but by the time it starts to appear in texts (King Alfred’s is the first recorded use, in his translation of Boethius’s De consolatione philosophiae around 888) it already meant ‘perpetually’. Alway survived into modern English, albeit as an archaism, but began to be replaced as the main form by always in the 12th century.
The final -s is genitive, not plural, and was originally added to all as well as way: alles weis. It has a generalizing force, much as in modern English one might say of a morning for ‘every morning’. => way
mid-14c., compound of Old English phrase ealne weg "always, quite, perpetually," literally "all the way," with accusative of space or distance, though the oldest recorded usages refer to time; see all + way (n.). The adverbial genitive -s appeared early 13c. and is now the standard, though the variant alway survived into 1800s. OED speculates allway was originally of space or distance, "but already in the oldest Eng. transferred to an extent of time."