Come tradurreste in italiano ?
In inglese ho trovato questa spiegazione che probabilmente e' corretta, ma non trovo una traduzione adeguata in italiano:

Typically it's a line that someone plays religiously with a very high success rate that usually is some form of sideline. Not a sideline at move 2, like Anti-Sicilians, but maybe some novelty, like say, the line from "Dangerous Weapons: The Sicilian", in the Taimanov, that goes 1.e4 c5 2.Nf3 e6 3.d4 cxd4 4.Nxd4 Nc6 5.Nc3 Qc7 6.Be2 a6 7.Be3 Nf6 and now 8.a3!?

The "norm" or "Main Line" is 8.O-O, but someone that plays 8.a3 relgiously with great success could say that this is their "pet line".

Playing 30 moves of Main Line Najdorf with 6.Bg5, even if it gives you high success, would not be deemed one's "pet line". There is always something offbeat (though not necessarily extravegant) about it when referring to one's "pet line".