A spoken colloquialism that existed
pre-internet, despite what many people claim.
"Prolly" is a clipped pronunciation of "probably"; compare with "g'day" as a clipped pronunciation of "good day", or "gonna" as a common spoken shortening of "going to".
Certainly
non-standard, but not necessarily indicative of
the writer's laziness. For some, writing in this manner mimics their natural speech pattern/dialect.