Obviously a shortened word for 'probably' now used in
internet conversations. Before the advent of this usage, it was commonly used in southern U.S. states in particular Texas as common vernacular and does NOT indicate laziness but dialectal and regional verbage.
Using 'prolly' is not a sign of idiocy or laziness but apropos savoir faire, subcultural quirks, accents and mannerisms just like many other phrases that come out of regional places.
Just think
Pigeon English in Hawaii,
Brooklyn slang or English/British expressions....we all got 'em!