The made-up slang word boolin’ comes from the American gang culture, which basically means “chilling” or “hanging out” with someone or somewhere.
As mentioned before, the word is made-up by non-other than the primarily African American gang (organisation).
The idea behind the creation of the word is that the gang name Blood start with a B and their rivals’ name (Crips) starts with a C, so with that logic, any word that starts with (or contains) the letter C can be swapped.
This is to show superiority to their rival street gang the Crips, the word slang word Coolin’ was changed to Boolin’.
The first mention of the word on the internet was in 2005 when the explation/meaning was uploaded to the website Urban Dictionary.
Give its origin, the word was mostly used on the streets in the beginning as a form of identification for fellow gang members.
With the evolution of technology and the increase in the number of people using twitter, it has started appearing more and more after 2015.
Its use is not that widely used on the internet.