'untidy' originates from Old English/English composition: the negative prefix 'un-' (Old English) combined with 'tidy', which comes from Old English 'tidig' where 'tid' meant 'time' or 'season'.

'tidig' in Old English became Middle English 'tidi'/'tidy' and developed into the modern English adjective 'tidy'; the word 'untidy' was formed by prefixing 'un-' to 'tidy' to indicate the opposite quality.

Originally, elements related to 'tid' referred to 'time' or 'seasonal appropriateness'; 'tidy' evolved to mean 'neat' or 'well-arranged', and 'untidy' consequently came to mean 'not neat' or 'disordered'.