'expanse' originates from Latin, specifically the past-participle element 'expansus' (from 'expandere'), where 'ex-' meant 'out' and 'pandere' (or 'pendere' variant) meant 'to spread'.

'expanse' passed into English via Old French/Late Latin influence (Old French 'expanse' / Late Latin 'expansus') and appeared in Middle English before becoming the modern English word 'expanse'.

Initially it meant 'a spreading out' (the action or result of spreading); over time it came to denote the thing produced by that spreading — a 'wide area' or 'extent' — which is the modern sense.