That sign would actually have been originally on the inside of the door of a men's lavatory. It means 'dress' as in 'clothing' and is basically a slightly coyly-worded reminder to men that they should check they have rebuttoned their fly before leaving the loo :-) Similar I suppose to the modern-day signs saying "have you remembered to wash your hands?" that you frequently see on the insides of door leading out of toilet blocks.