A noun. It is the harmonious unification of the words "out" and "bro". It basically means "I'm out, bro." In other words, "I'm out, brother." In other words, "I'm leaving, my friend."
What!?!?! J.Lo is NOTgoing to be performing? I'm outtro!
An outroll or "outro troll" occurs during the end of a video, song, monologue, or other media that contains a final or departing humorously insulting jab or rib of or to another party, entity, product, or person. See also "introll."