“Cannot try”  or “Cannot try. Dirty how?”  are probably most used by sales assistants in apparel shops. Both are grammatically wrong and it isn’t very pleasant for customers to be turned down this way.

“Dirty how?”  as a question will also not make any sense to foreigners who aren’t familiar with the non-standard English phrases common in Singapore.

Help Your Customer To Understand Your Company Policy

It is for practical reasons that customers are not allowed to try on the items on sale.  But that doesn’t give the sales assistants the right to be rude when declining the request.

It is better to say, “I’m sorry but we do not allow customers to try these on for hygiene reasons.”

Source: Edited from the Singapore Retailers Association's Speak Good English pamphlet.