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."