'undiplomatic' originates from English, specifically formed from the prefix 'un-' and the adjective 'diplomatic', where 'un-' meant 'not' (a negation prefix) and 'diplomatic' derived from French 'diplomatique' and Greek 'diplōma' meaning 'folded paper' or 'official document'.

'diplomatic' changed from French 'diplomatique' (and via Latin/Greek 'diplōma') into Middle English forms relating to official documents and envoys, and later developed the sense of tact or skill in relations; 'undiplomatic' was formed in English by adding the negating prefix 'un-' to that adjective.

Initially related to 'diplōma' as a 'folded paper' or official document, the word evolved to describe official envoys and their conduct ('diplomat'), then to the quality of handling relations with tact; 'undiplomatic' came to mean 'not tactful' or 'lacking diplomatic skill'.