A fork of Rural Dictionary
Literary colloquialism for "probably", most likely first used in print in John Kennedy Toole's Confederacy of Dunces, (1980), in the speech of one of the book's characters, the mother of the protagonist, Ignatius J. Reilly. The use of the word is meant to reflect the speech typical of white working-class residents of New Orleans, Louisiana.
That prolly is the reason he left in the first place.
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!
Y'all prolly think I'm dumb for using this but it ain't so. I just wanna get to the point faster without havin' ta use so many goddamn letters! ;D