'aslumber' originates from Middle English, formed from the prefix 'a-' (meaning 'on, in') attached to the noun 'slumber' (meaning 'sleep').

'aslumber' developed in Middle English (e.g., forms like 'aslomber'/'a-slumber') from '-slumber', which itself goes back to older Germanic roots for 'sleep', and it retained its form into modern English as 'aslumber'.

Initially it conveyed the literal sense 'in slumber' and over time has remained largely the same, though the word has become somewhat archaic/poetic in modern usage.