'non-diplomatic' originates from the prefix 'non-' meaning 'not' and the word 'diplomatic', which comes from the Greek word 'diplōmatikos', meaning 'pertaining to official documents'.

'diplomatic' evolved from the Greek 'diplōmatikos' through Latin 'diplomaticus', and eventually became the modern English word 'diplomatic'. The prefix 'non-' was added to form 'non-diplomatic'.

Initially, 'diplomatic' meant 'pertaining to official documents', but over time it evolved to mean 'skilled in dealing with sensitive matters'. 'Non-diplomatic' retains the negation of this evolved meaning.