'antidiphtherical' originates from Greek and New Latin elements: the prefix 'anti-' (Greek) meaning 'against' combined with 'diphtheria' (from Greek 'diphthera' meaning 'leather, hide'), plus the English adjectival suffix '-ical'.

'antidiphtherical' developed in English in the late 19th century as a formation from 'antidiphtheric' (a similar adjective formed from 'anti-' + 'diphtheria'), with the later adoption of the '-ical' variant to form 'antidiphtherical'.

Initially it meant 'acting against diphtheria' in a literal medical sense, and over time it has retained that specific medical meaning.