debility: [15] Despite the passing similarity, debility has no connection with ability. It comes via Old French debilite from Latin dēbilitās, a derivative of the adjective dēbilis ‘weak’. This was a compound formed from the prefix de- ‘not’ and a second element meaning ‘strong’, represented also in Sanskrit bálam ‘strength’, Greek beltíon ‘better’, and Old Slavic bolij ‘larger’ (ultimate source of bolshevik). [debility etymology, debility origin, 英语词源]