'resupply' is formed from the prefix 're-' (meaning 'again', ultimately from Latin 're-') + 'supply'. 'Supply' originates from Latin 'supplēre', where 'sup-' (or 'sub-/sup-') meant 'up to' or 'from below' and 'plēre' meant 'to fill'.

'supply' passed into English via Old French and Middle English (e.g. Middle English 'supplyen') from Latin 'supplēre'. The modern term 'resupply' was formed in English by adding the productive prefix 're-' to 'supply' to indicate doing the action again.

The Latin root originally meant 'to fill up'; over time the sense broadened to providing or furnishing what is needed, and 'resupply' developed the specific modern sense 'to provide again or replenish'.