1. an uncompromising nature concealed by an unassuming appearance or disposition
don't be fooled, he/she is an iron fist in a velvet glove
This expression can be used as either a compliment or criticism. As a compliment, it can describe somebody whose outward timidity or weakness conceals an inner strength or fierce resolve, or somebody who combines likability with uncompromising decisiveness. As a criticism, it can describe somebody who feigns kindliness in order to exploit or deceive, or somebody who lets power and authority go to their head. It is used in both a social and professional setting.