Green’s Dictionary of Slang

tailgate v.

[SAmE tailgate, the tailboard of a truck or wagon]

1. [1950s+] to drive a car closely (too closely) behind the one in front; thus tailgating n.

2. [1970s+] also in fig. use, to follow, esp. of things that are endlessly repetitive.

3. [1990s+] (US campus) to watch women passing by.