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!