So tell Katy Perry he's on her tail, he's tailgating

from Bad Meets Evil – A Kiss Lyrics on Rap Genius


On the tail means following her secretly so tailgating means that he is following or stalking her very closely ( tailgate means to drive close behind someone)
Also, “on the tail” could mean he’s literally on her tail, AKA her ass, he’s “tailgating,” or basically fucking her in the ass

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