This is a colloquial version of the word "probably", in common usage in the upper Midwest, especially those areas of Wisconsin whose populations were drawn from German-speaking immigrants like Milwaukee and Madison. Definitely predates Internet.

Short way of saying probably. Lazy people or people that just can't spell "probably" will use this shortened version.

Prolly is another word for probably. You will most likely see teenagers use this a lot more than adults because it’s not known to be “proper grammar”.

The way fags and fucks say or type the word probably.

Short for the term "probably"

Short for probably. Used by only OG mofo's, Cholo's and gangbangers

Internet shorthand for probably.