Gandi Baat [Devanagari:गन्दी बात, Urdu:گندی بات] is a Hindi/ Urdu phrase which simply means 'bad thing'. The phrase is used in both Hindi and Urdu with almost equal ease and commonness.
Gandi baat is a phrase that is commonly spoken in families to kids when they do anything wrong, to tell them they're doing something bad which they shouldn't and so has a slight innocence attached to it.