'That’s her. That’s the lady lawyer I was talking about.'
'Man, that is a fine-looking woman.'
'When you said she was a lawyer I was thinking of someone playing on a rubber tyre at the end of a chain. But that lady is hot, boss.'
Everton was right. The woman had more curves than a bag full of footballs.
from False Nine by Philip Kerr
With the additional information above there are enough textual clues to understand that the words "someone playing on a rubber tyre at the end of a chain" refers to a girl who is young or looks young, whose body is not developed, boy-like. He means he assumed that this lawyer was a woman who wasn't physically beautiful and didn't have a lot of prospects to marry well so she worked hard and became a lawyer to be able to provide an income for herself. But, actually, the lady lawyer is very attractive (as well as intelligent).