Queer in standard English means odd or strange.
It has also been used as a derogatory slang word for a male homosexual, particularly in the mid-20th Century.
More recently, however, the term queer has become acceptable within the Lesbian, Gay, Bisexual and Transgender communities. It may, therefore, sometimes be used as an alternative to the LGBT designation.
As with the word gay, queer is often used in Tipping the Velvet in an ambiguous way.