Derived from the term "propylene". No one really knows what propylene really is, but everyone assumes that it is a hair product, such as grease. However, upon googling the term "propylene", it is said to be a dangerous chemical, but this is irrelevant to the subject in question. Pro-pro is a shortened version of this word, simply meaning hair grease or gel. Pro-pro is also the act of styling or washing hair in any manner. Pro-pro is synonymous to hair.
"Gurl, why you puttin' so much pro-pro on yo' head? Didn't you just get yo hurr did?"