In the play set in Denmark that Pip and his companions attend in Chapter 31, Dickens uses a humorous idiom that also functions as a simile, likening the character of the Queen of Denmark to the metal she wears:
The Queen of Denmark, a very buxom lady, though no doubt historically brazen, was considered by the public to have too much brass about her; her chin being attached to her diadem by a broad band of that metal (as if she had a gorgeous toothache), her waist being encircled by another, and each of her arms by another, so that she was openly mentioned as “the kettle-drum.”
Being considered "brassy" or "brazen" would have been a criticism for a Victorian woman, as both words mean "loud" and "obvious" in this context. Being polite, quiet, and retiring was considered to be proper behavior for ladies at this time, so the "brass" in this woman is not necessarily a kind judgement by Pip. Dickens links the real brass the actress is wearing here to the "historically brazen" character she plays. The passage uses a second simile to make her seem even more comical, as Pip tells the reader she was "openly mentioned as the kettle-drum" because of all the rings of brass on her body.