Indiatimes chat with Aishwarya:

Ritika: Whose thought was ‘Ish’ in Devdas?
Aishwarya Rai: Definitely Sanjay Leela Bhansali’s because it is a very endearing term in Bengali. So we all found it very charming and we even continued with it long after the film (shooting) was over.

Hmm. Was a little too endearing for my taste — seems like the director liked the sound of the word and got carried away with it :-). ‘Ish’ in Bengali roughly translates to ‘tsk tsk’ in English, with deeper connotations of regret, or in a humorous situation, humour.