'unprincipled' originates from the prefix 'un-' meaning 'not' and the word 'principled,' which comes from the Latin word 'principium,' meaning 'beginning' or 'foundation.'

'Principled' evolved from the Latin 'principium' through Old French 'principe' and Middle English 'principle,' eventually forming 'principled' in modern English. The prefix 'un-' was added to denote the opposite.

Initially, 'principled' meant 'having a foundation or beginning,' but over time it evolved to mean 'having moral principles.' Adding 'un-' reversed this meaning.