Wordorigins.org

The origin of this word for a prostitute is unknown. But while we don’t know the origin for sure, there are a couple likely explanations, the most likely being that it is simply a reference to the prostitute’s ability to snare, or hook, clients. And we do know for sure that the story of hooker being an eponym for a Civil War general is false.

The earliest appearance of the word is in Norman E. Eliason’s Tarheel Talk in a citation from 1845:

If he comes by way of Norfolk he will find any number of pretty Hookers in the Brick row not far from French’s hotel.