The shortest possible abbreviation of "Cool stroy bro."
"Cool story bro" itself is said to someone when they say something which is often boring or unrelated to the topic of conversation.
Distaste and disinterest in the topic is implied through the sarcastic remark, "cool story bro". Of course, the story was not cool - that is the point.
"Cool story bro" was changed to "Brool story co" in order to add slightly more variety to an over-used phrase. This became contracted to simply "brool".
John: "did you see the match yesterday? What a goal!"
Tom: "nah, saw the highlights though - keeper looked awful."
James: "i like cats"
Either Tom or John: "brool".
The initial conversation then resumes.