I suspect you mean 'a Don Juan'...ie the Spanish name which is pronounced almost as 'one'. Don Juan was a legendary Spanish nobleman who was very licentious - keen on the ladies, in other words - in various European plays, poems and operas. As a result, the name is nowadays applied to any such man.
Although 'Don' used to mean nobleman, (Don Guiseppe is another example) it now is generalised to all man in Spain or Italy. In the same way only noble Englishman could have the title 'esquire' after their name, now the title is used for all men. Donna is the female version, as in the phrase 'prima donna'.