'undependable' originates from the prefix 'un-' meaning 'not' and the word 'dependable', which comes from the Latin 'dependere', meaning 'to hang down from'.

'dependable' evolved from the Latin 'dependere' through Old French 'dependre', and eventually became the modern English word 'dependable'. The prefix 'un-' was added to form 'undependable'.

Initially, 'dependable' meant 'to hang down from', but over time it evolved into its current meaning of 'reliable'. Adding 'un-' negates this meaning.