Green’s Dictionary of Slang

tail v.

1. [late 18C+] to have sexual intercourse with, to work as a prostitute.

2. [mid-19C; 20C+] to follow, to keep under (police) surveillance.

3. [1930s] to make the object of a criminal plan.

In derivatives

tailer (n.)

[1910s] (US und.) a bodyguard who accompanies the transporter of valuable goods.