'expel' originates from Latin, specifically the word 'expellere', where 'ex-' meant 'out' and 'pellere' meant 'to drive'.

'expellere' passed into Medieval Latin and Old French (e.g. 'espulser' / 'expulsare') and then into Middle English as 'expellen', eventually becoming modern English 'expel'.

Initially it meant 'to drive out' or 'force away'; over time the usage broadened but the core idea of driving or forcing something out has largely remained (including ejecting substances or bodily waste).