'appender' originates from the verb 'append', ultimately from Latin 'appendere', where 'ad-' meant 'to, toward' and 'pendere' meant 'to hang.'

'append' moved from Latin 'appendere' into Late Latin/Old French forms and Middle English (e.g. Middle English 'appenden'), and the agentive suffix '-er' was later added in English to form 'appender' meaning 'one that appends.'

Initially it meant 'to hang upon or attach,' but over time it evolved into the sense 'to add or attach (often data) at the end,' which is common in modern use.