"then tipped the velvet"
Tipping the Velvet
Public DomainTipping the Velvet - Credit: Mihály Zichy
This is the first reference to the title of the book. Nan King, like many readers no doubt, is not familiar with the term tipping the velvet.

It is defined in the Dictionary of the Vulgar Tongue (1811) as tonguing women, and in the Dictionary of Victorian Sexual Slang as tonguing a woman's mouth, or if you were very naughty,  her muff.