'baneful' is formed in English from the noun 'bane' + the suffix '-ful', with 'bane' meaning a cause of death or ruin.

'bane' comes from Old English 'bana' meaning 'slayer' or 'cause of death'; Middle English kept the form 'bane', and the adjective 'baneful' developed by adding the suffix '-ful' in later Middle English/early Modern English.

Originally related to literal killing or causing death ('slayer'/'cause of death'), the sense broadened to mean causing ruin, serious harm, or pernicious effects in general.