'unteacherly' originates from Modern English as a formed adjective combining the prefix 'un-' + 'teacherly', where 'un-' meant 'not' and 'teacher' meant 'one who teaches'.

'teacher' comes from Old English 'tǣċere' (from the verb 'tǣċan', to teach). The adjective-forming suffix '-ly' produced 'teacherly', and later the negative prefix 'un-' was added to create 'unteacherly'.

Initially, the formation simply meant 'not in the manner of a teacher' (literally 'not teacherly'); over time it has been used to describe conduct or qualities considered unprofessional or unbecoming of a teacher.