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!