deaf: [OE] Ultimately, deaf and dumb come from the same source, and moreover they are related to a Greek word for ‘blind’. The common denominator ‘sensory or mental impairment’ goes back to an Indo-European base *dheubh-, which denoted ‘confusion, stupefaction, dizziness’. It produced Greek tuphlós ‘blind’; English dumb; and a prehistoric Germanic adjective *daubaz ‘dull, stupefied, slow’.
Many of the modern descendants of *daubaz retain this general sense – Danish doven means ‘lazy’ – but English has specialized it to ‘dull in hearing’. Duffer may ultimately be derived from Old Norse daufr ‘deaf’ in which the sense ‘dull, stupid’ is preserved. => duffer, dumb
