This is the sixth outing for Justin Timberlake and Jimmy Fallon and their "History of Rap" performance series, but who's counting? It never gets old.

Justin was part of an all-star guest lineup on "The Tonight Show" last night (Sept. 9), kicking off the show with a medley homage to the greatest rap and hip hop songs of all time.

This time around, the mashup had a distinct (and timely) "Straight Outta Compton" vibe, but the real blast from the past is when Jimmy drags the tongue-twisting "Informer" by Canadian reggae musician Snow back into circulation after more than 20 years -- and learns an important lesson about when it's appropriate to licky your boom-boom down.

(Hint: Never. It's never appropriate.)